Typo in 訊號進出場(hold_until)

https://doc.finlab.tw/details/backtest_decorator/#hold_until

# 訊號進出場(hold_until)
## 這是所有策略撰寫中，最重要的語法糖，用法為：
```
from finlab import data

close = data.get('price:收盤價')
buy = close > close.averate(5)
sell = close < close.average(20)

position = buy.hold_until(sell)
```
Should be ->

```
buy = close > close.average(5)
``` 