Change monospace text formatting

Summary:
Using ## can cause some formatting issues, for example:

{F453436}

As such, use ` instead.

Test Plan: Generated documentation locally.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13071