Bug 1561102 - Add mach crate to license.html as well. r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D35871

--HG--
extra : moz-landing-system : lando

toolkit/content/license.html

@@ -2904,6 +2904,7 @@ SOFTWARE.
         <li><code>third_party/rust/arrayref</code></li>
         <li><code>third_party/rust/cloudabi</code></li>
         <li><code>third_party/rust/Inflector</code></li>
+        <li><code>third_party/rust/mach</code></li>
     </ul>
     See the individual LICENSE files for copyright owners.</p>