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)
