removing superfluous reference to axis in Series.reorder_levels docst…
Hi,
I removed the superfluous reference to axis in Series.reorder_levels docstring.

- [ ] xref #22627
- [x] tests added / passed (doesn't apply)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry (doesn't apply, only removed a line in the docstring)

Hello @SandrineP! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/series.py`](https://github.com/SandrineP/pandas/blob/f1bb2d82da19c2f4c371988a2643859e4104e272/pandas/core/series.py) !

 This is good for now. I've changed the PR message to not close #22627, in case someone wants to do the refactor. Thanks @SandrineP 