fix infer_freq raises section
Fixes the raises section of the `infer_freq` doc string. Seems small enough to skip a whats new entry.

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


Thanks @wholmgren 