Fixed Typos in docstrings.rst

[hobojoe1848]

There were two instances where the term docstring was written as docsting.

Nothing major but given we copied and pasted verbatim for another purpose, we managed to copy the typo across by accident.

Fixing and PRing so we don't make the same mistake again :)

[sglvladi]

@hobojoe1848 Thanks for your contribution!