Fix the return type declaration

See https://www.woltlab.com/community/thread/309479-falscher-phpdocs-kommentar-im-ilookuppagehandler/

wcfsetup/install/files/lib/system/page/handler/ILookupPageHandler.class.php

@@ -34,7 +34,7 @@ public function isValid($objectID);
      * an `objectID => title` relation.
      *
      * @param string $searchString search string
-     * @return  string[]
+     * @return  string[][]
      */
     public function lookup($searchString);
 }