Multiple indicators example throws an error

The [multiple indicators example](https://github.com/alpacahq/alpaca-backtrader-api/blob/master/sample/strategy_multiple_indicators.py) throws an error with the following config and real API keys

```
IS_BACKTEST = False
IS_LIVE = True
symbol = "AAPL"
```

```
$ python live_v3.py
2021-06-28 03:04:49,918 started trading stream
Traceback (most recent call last):
File "/Users/jonathanrhone1/Src/quant/live_v3.py", line 127, in
cerebro.run()
File "/Users/jonathanrhone1/miniconda3/envs/quant39/lib/python3.9/site-packages/backtrader/cerebro.py", line 1127, in run
runstrat = self.runstrategies(iterstrat)
File "/Users/jonathanrhone1/miniconda3/envs/quant39/lib/python3.9/site-packages/backtrader/cerebro.py", line 1187, in runstrategies
self._broker.start()
File "/Users/jonathanrhone1/miniconda3/envs/quant39/lib/python3.9/site-packages/alpaca_backtrader_api/alpacabroker.py", line 103, in start
self.positions = self.update_positions()
File "/Users/jonathanrhone1/miniconda3/envs/quant39/lib/python3.9/site-packages/alpaca_backtrader_api/alpacabroker.py", line 83, in update_positions
broker_positions_symbols = [p.symbol for p in broker_positions]
File "/Users/jonathanrhone1/miniconda3/envs/quant39/lib/python3.9/site-packages/alpaca_backtrader_api/alpacabroker.py", line 83, in
broker_positions_symbols = [p.symbol for p in broker_positions]
File "/Users/jonathanrhone1/miniconda3/envs/quant39/lib/python3.9/site-packages/alpaca_trade_api/entity.py", line 29, in __getattr__
return super().__getattribute__(key)
AttributeError: 'Position' object has no attribute 'symbol'
```

```
$ pip freeze
alpaca-backtrader-api==0.14.0
alpaca-trade-api==1.2.2
backtrader==1.9.76.123
pandas==1.1.5
```

I also had to comment the `Starting Portfolio Value` line.