js: Fix util_translate_time_string() usage

util_translate_time_string() was called to use LC_TIME to translate
strings, but those strings were not marked as to be translated.

https://bugzilla.gnome.org/show_bug.cgi?id=739822

js/ui/dateMenu.js

@@ -129,7 +129,7 @@ const DateMenuButton = new Lang.Class({
                 /* Translators: This is the date format to use when the calendar popup is
                  * shown - it is shown just below the time in the shell (e.g. "Tue 9:29 AM").
                  */
-                let dateFormat = Shell.util_translate_time_string ("%A %B %e, %Y");
+                let dateFormat = Shell.util_translate_time_string (N_("%A %B %e, %Y"));
                 this._date.set_label(now.toLocaleFormat(dateFormat));
             }
         }));