DOC: Update documentation for pd.Interval if string endpoints are not allowed anymore
- [X] closes #36002 
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Removed outdated example in documentation for pd.Interval (string endpoints are not allowed anymore).