Bug 661426 - Fix wrong style sheet name in test_xulbrowser_plugin_visibility.xul. r=josh

dom/plugins/test/mochitest/test_xulbrowser_plugin_visibility.xul

@@ -1,6 +1,6 @@
 <?xml version="1.0"?>
 <?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
-<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.cs" type="text/css"?>
+<?xml-stylesheet href="chrome://mochikit/content/tests/SimpleTest/test.css" type="text/css"?>
 
 <window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
   <script type="application/javascript"