Two missing spaces in documentation
I fixed the missing space that made the documentation render "isthe" instead of "is the".

Before the change:
- This **isthe** equivalent of the numpy.ndarray method argmax.
- This **isthe** equivalent of the numpy.ndarray method argmin.

After the change:
- This **is the** equivalent of the numpy.ndarray method argmax.
- This **is the** equivalent of the numpy.ndarray method argmin.


- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry
