Update event_dispatcher.rst

Update code example to match 5.1 PHP configuration format.

Author: yivi

event_dispatcher.rst

@@ -101,8 +101,10 @@ using a special "tag":
 
         // config/services.php
         use App\EventListener\ExceptionListener;
+        
+        $services = $containerConfigurator->services();
 
-        $container->register(ExceptionListener::class)
+        $services->set(ExceptionListener::class)
             ->addTag('kernel.event_listener', ['event' => 'kernel.exception'])
         ;