R1:3c984eec56d0

Fix for escaping character "'".

Summary:
The position of the custom rule was incorrect.

Test Plan:
Tested that Facebook task remarkup, Differential remarkup, image macros, quotes,
and random characters were working correctly in comment preview

Reviewed By: jungejason
Reviewers: jungejason
CC: aran, jungejason
Differential Revision: 396