Fix escaped HTML in burnup report
Summary: Fixes T4378. I just removed the <em> since this element is unusual and it's not convenient to switch it to translatable remarkup.
Test Plan: stared at it real good
Reviewers: btrahan
Reviewed By: btrahan
CC: aran, chrisbolt
Maniphest Tasks: T4378
Differential Revision: https://secure.phabricator.com/D8155