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