shellDBus: Fix SetActive(false) for ScreenSaver DBus API

Someone changed .unlock() to deactivate() for the obvious symmetry
with activate(), but forgot to update it here.

https://bugzilla.gnome.org/show_bug.cgi?id=696585

js/ui/shellDBus.js

@@ -397,7 +397,7 @@ const ScreenSaverDBus = new Lang.Class({
         if (active)
             this._screenShield.activate(true);
         else
-            this._screenShield.unlock(false);
+            this._screenShield.deactivate(false);
     },
 
     GetActive: function() {