logout dialog: Show the correct text right away

The end session dialog was waiting a second before updating
its text to display the timer. It is nicer to show the correct
message from the start.

https://bugzilla.gnome.org/show_bug.cgi?id=702056

js/ui/endSessionDialog.js

@@ -420,6 +420,7 @@ const EndSessionDialog = new Lang.Class({
     _startTimer: function() {
         let startTime = GLib.get_monotonic_time();
         this._secondsLeft = this._totalSecondsToStayOpen;
+        this._updateDescription();
 
         this._timerId = Mainloop.timeout_add_seconds(1, Lang.bind(this,
             function() {