Issue 4300 - Fix typo in $sitekey adding logic

include.preload.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 159 matching lines @@
           );
       }
     }
   );
 }
 
 function checkSitekey()
 {
   var attr = document.documentElement.getAttribute("data-adblockkey");
   if (attr)
-    ext.backgroundPage.sendMessage({type: "filter.addKey", token: attr});
+    ext.backgroundPage.sendMessage({type: "filters.addKey", token: attr});
 }
 
 function getContentDocument(element)
 {
   try
   {
     return element.contentDocument;
   }
   catch (e)
   {
@@ skipping 373 matching lines @@
   }, true);
 
   return updateStylesheet;
 }
 
 if (document instanceof HTMLDocument)
 {
   checkSitekey();
   window.updateStylesheet = init(document);
 }