Fix remarkup "NOTE" block syntax

Summary: Currently, HTML in NOTE blocks gets escaped, because the "rtrim()" applies to a PhutilSafeHTML object and implicitly converts it to (unsafe) string. Instead, trim first.

Test Plan: Typed `NOTE: **x**` into remarkup, saw bold "x".

Reviewers: btrahan, garoevans, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6379