Maniphest - make instructions !assign savvy

Summary: forgot this in last diff.  Ref T3937.

Test Plan: looks good

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T3937

Differential Revision: https://secure.phabricator.com/D7308

src/applications/maniphest/mail/ManiphestReplyHandler.php

@@ -27,8 +27,8 @@ final class ManiphestReplyHandler extends PhabricatorMailReplyHandler {
 
   public function getReplyHandlerInstructions() {
     if ($this->supportsReplies()) {
-      return "Reply to comment or attach files, or !close, !claim, or ".
-             "!unsubscribe.";
+      return "Reply to comment or attach files, or !close, !claim, ".
+             "!unsubscribe or !assign <username>.";
     } else {
       return null;
     }