DOC: Remove extra backtick in example in documentation.
:meth:`~Series.idxmax``  --> :meth:`~Series.idxmax`

- [x] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
