DOC: Fix EX02 in pandas.Series.factorize
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Related to #27977. 

output of `python scripts/validate_docstrings.py pandas.Series.factorize`:
```
################################################################################
################################## Validation ##################################
################################################################################
```


> 
> 
> lgtm, and CI is green. Is this working for you locally @farhanreynaldo ?

Yes, it's working now. Thanks @farhanreynaldo !