Update performance.rst

The example provided under "Caching the Autoloader with APC" is not in sync with the content of the 'app.php' file in Symfony 2.8:
* the comment hint was removed in #964
* the autoloader was extracted from bootstrap: symfony/symfony-standard@747a384

Author: cristi-contiu

performance.rst

@@ -71,21 +71,21 @@ Caching the Autoloader with APC
 Another solution is to cache the location of each class after it's located
 the first time. Symfony comes with a class - :class:`Symfony\\Component\\ClassLoader\\ApcClassLoader` -
 that does exactly this. To use it, just adapt your front controller file.
-If you're using the Standard Distribution, this code should already be available
-as comments in this file::
+If you're using the Standard Distribution, make the following changes::
 
     // app.php
     // ...
+    
+    use Symfony\Component\ClassLoader\ApcClassLoader;
 
-    $loader = require_once __DIR__.'/../app/bootstrap.php.cache';
+    $loader = require __DIR__.'/../app/autoload.php';
+    include_once __DIR__.'/../app/bootstrap.php.cache';
 
     // Use APC for autoloading to improve performance
     // Change 'sf2' by the prefix you want in order
     // to prevent key conflict with another application
-    /*
     $loader = new ApcClassLoader('sf2', $loader);
     $loader->register(true);
-    */
 
     // ...