error encountered in yahoo downloader in the very first tutorial notebook

Hi,

I tried running the tutorial in Google colab (the first tutorial notebook), and everything at the beginning ran without errors until this line

```
df = YahooDownloader(start_date = TRAIN_START_DATE,
end_date = TRADE_END_DATE,
ticker_list = config_tickers.DOW_30_TICKER).fetch_data()

```

I'm getting the following error:

```
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- AXP: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- AMGN: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- AAPL: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- BA: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- CAT: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- CSCO: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- CVX: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- GS: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- HD: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- HON: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- IBM: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- INTC: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- JNJ: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- KO: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- JPM: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- MCD: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- MMM: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- MRK: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- MSFT: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- NKE: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- PG: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- TRV: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- UNH: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- CRM: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- VZ: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- V: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- WBA: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- WMT: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- DIS: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- DOW: No data found for this date range, symbol may be delisted
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
[](https://localhost:8080/#) in ()
----> 1 df = YahooDownloader(start_date = TRAIN_START_DATE,
2 end_date = TRADE_END_DATE,
3 ticker_list = config_tickers.DOW_30_TICKER).fetch_data()

[/usr/local/lib/python3.9/site-packages/finrl/meta/preprocessor/yahoodownloader.py](https://localhost:8080/#) in fetch_data(self, proxy)
58 num_failures += 1
59 if num_failures == len(self.ticker_list):
---> 60 raise ValueError("no data is fetched.")
61 # reset the index, we want to use numbers as index instead of dates
62 data_df = data_df.reset_index()

ValueError: no data is fetched.

```

Is `yahooDownloader` broken?