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

> lgtm, thanks @MomIsBestFriend 
> 
> I think in a follow up would be good to have a single call for all the doctests. Not sure why we group few cases, then call some files individually...
> 
> Also, if you keep fixing tests, may be you can fix one directory at a time. So, for example, instead of having to call evey file in `pandas/core/arrays` separetely, we can just have one call for the whole directory.
> 
> Ideally, at some point we simply want to have a call to `pytest -q --doctest-modules pandas`
> 
> Thanks!

@datapythonista Exactly, this is what I am aiming for.

And about the reason why there are "groupings" of, some of them are there for legacy reasons and the other reason is to have it as a "placeholder", I was afraid that even if a file is fixed, overtime it will become "wrong" agian, so this is just putting things in the "safe" place. ping @simonjayhawkins  Thanks @MomIsBestFriend 