Remove duplicate entry on series.rst
- [x] closes #38198 
- [ ] tests passed. Nothing new fails. Few tails already were failing on `master`
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry (Intentionally ignored as insignificant)

Removed the duplicate entry of `Series.replace` from doc
thanks @vikramaditya91 