DOC: Fixed docstring in Series .isin() method
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Changed the docstring in `Series.isin()` (`def isin(self, values) -> "Series":`)
from 

> Check whether `values` are contained in Series.

to 

> Whether elements in Series are contained in 'values'. Thanks, @mariadelmarbibiloni 