DOC: Fix small spelling mistake in style docs
This pull request fixes a small spelling mistake in the pandas style user guide.

I believe this change does not justify a whatsnew entry, since it is extremely small. It does not refer to a particular issue on GitHub. I have not run `black` or `flake8`, since I believe the change is out of scope for these tools.

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

Thanks @flrs 