[fbsync] Fix command to install the sphinx theme for doc building (#4833)

Summary:
Fix for #4832

The unauthenticated git protocol on port 9418 is no longer supported.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.

Reviewed By: kazhang

Differential Revision: D32216695

fbshipit-source-id: 2456b46d516246377034b48c7896799f6c8c0cd8

Author: prabhat00155

docs/requirements.txt

@@ -3,4 +3,4 @@ numpy
 sphinx-copybutton>=0.3.1
 sphinx-gallery>=0.9.0
 sphinx==3.5.4
--e git+git://github.com/pytorch/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme
+-e git+https://github.com/pytorch/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme