Update contribution_guide.rst (#36438)
Summary:
Fix formatting: change "Frequently Asked Questions" into an RST header, which is clickable and one get a URL of the FAQ section
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36438
Differential Revision: D21106180
Pulled By: mruberry
fbshipit-source-id: 370dafd1883bd57285b478cf2faa14ae2f86e3ba
