Fix line_terminator escaping in python doc
The line terminators are not escaped so we don't see the backslash here:

![image](https://user-images.githubusercontent.com/9317502/113784381-9ee80a00-9735-11eb-8e5e-6d4b84da7527.png)

Source : https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_csv.html
thanks @Drarig29 