Baseline code doesn't match paper numbers

**Describe the bug**
Baseline numbers don't quite match paper numbers. They're close, but not sure why they aren't exact... am I doing anything wrong?

**To Reproduce**
Steps to reproduce the behavior:
```
from finrl.plot import backtest_stats, get_baseline
start = "2016-01-04"
end = "2020-05-08"
baseline_df = get_baseline(ticker="^DJI", start = start, end = end)
stats = backtest_stats(baseline_df, value_col_name = 'close')
print(stats)
```

**Expected behavior**
Copied from paper: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3690996
```
Annual return: 0.078
Cumulative returns: 0.386
Annual volatility: 0.201
Sharpe ratio: 0.47
Max Drawdown: -0.371
```

**Actual behavior**
```
Annual return 0.079210
Cumulative returns 0.392266
Annual volatility 0.200459
Sharpe ratio 0.481570
Max drawdown -0.370862
```