Change text color of "You can merge this merge request manually..." within MR widget

This issue is part of https://gitlab.com/gitlab-org/gitlab-ce/issues/33095

Design/Proposal

Change this:

to this:

Google_Chrome_2017-06-20_14-04-392

color: #333;

cc: @victorwu (making the https://gitlab.com/gitlab-org/gitlab-ce/issues/33095 more readable)