CLN GH23204 remove "not" typo in assertion
- [x] closes #23204 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Remove an unwanted "not" in the assertion message.

This is my first pandas PR! Let me know if there's anything else I need to do. Thanks!