Fix: Undefined variable passed to hook

* This problem was introduced in
  I6fad69b5d9b9ca8a7d12c7e410d3ae6180fbddbf
* The $render variable is undefined in that context
* $this should be used instead

Change-Id: I70e0a03e41f0312a458d1dd27b766452ae093093

MathMathML.php

@@ -483,7 +483,7 @@ class MathMathML extends MathRenderer {
 				$this->setMathml( $jsonResult->mml );
 			}
 			Hooks::run( 'MathRenderingResultRetrieved',
-				array( &$renderer,
+				array( &$this,
 					   &$jsonResult ) ); // Enables debugging of server results
 			return true;
 		} else {