Fix very odd wrapping / linebreaking for Remarkup code block headers in Safari

Summary: Fixes T13118. Ref T13120. This construction is a little odd; I'm not entirely sure why Safari is doing what it's doing, but this appears to fix it.

Test Plan: Viewed blocks like those in T13118 in Safari. Before the patch, weird last-letter wrapping. After the patch, sensible behavior.

Maniphest Tasks: T13118, T13120

Differential Revision: https://secure.phabricator.com/D19303