DOC: Fix wrongly associated docstring of Series.str.isspace
cc @simonjayhawkins

- [ ] closes #42015
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

thanks @felixdivo  @meeseeksdev backport 1.3.x Something went wrong ... Please have  a look at my logs.