Small fixes to the Contributing.txt (#63385)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63385

Correcting a mistake for the pytorch uninstall, and
adding an extra note for Darwin.

Test Plan: Imported from OSS

Reviewed By: janeyx99, heitorschueroff

Differential Revision: D30530234

fbshipit-source-id: e0f88a1725eeadabfb4b28c1da11e369ee878ab4