display: Mark "Mirror displays" as a verb

Add a translator comment to make sure "Mirror displays" in the main
panel UI gets translated properly.

panels/display/display-capplet.ui

@@ -224,7 +224,7 @@
                 <property name="orientation">vertical</property>
                 <child>
                   <object class="GtkCheckButton" id="clone_checkbox">
-                    <property name="label" translatable="yes">_Mirror displays</property>
+                    <property name="label" translatable="yes" comments="Note that mirror is a verb in this string">_Mirror displays</property>
                     <property name="visible">True</property>
                     <property name="can_focus">True</property>
                     <property name="receives_default">False</property>