DOC: pandas.Series.str.replace inconsistent parameter (regex)
### 

- [X] I have checked that the issue still exists on the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/)


### Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.Series.str.replace.html

### Documentation problem

The method description lists regex=None as the default, while the argument and examples list regex=True. In addition, the argument description does not list None as a possibility. 

### Suggested fix for documentation

Change method description to regex=True as that seems to be accurate behavior. 