DOC: Fix typo of `=!` to `!=` in docstring
This fixes GH36075.

- [x] closes #36075 

I'm marking the following list items as not-applicable because I'm simply swapping two characters in a docstring that had been in the incorrect order.

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