Fix the docstring of xs in pandas/core/generic.py #22892 
- [X] closes #22892
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @datapythonista !
As requested in #22892 , I corrected the docstring of the pandas/core/generic.py xs function to pass correctly the following command :

    ./scripts/validate_docstrings.py pandas.Series.xs

And I also validate the PEP-8 according to : 

    flake8 --doctests pandas/core/generic.py

Is it ok for you ?
