Use a more obvious unconventional class name. ExtensionHello looks like a typo.

Author: AD7six

cookbook/bundles/extension.rst

@@ -504,7 +504,7 @@ automatically by Symfony2. If not, override the Bundle
 :method:`Symfony\\Component\\HttpKernel\\Bundle\\Bundle::build` method in
 your bundle::
 
-    use Acme\HelloBundle\DependencyInjection\ExtensionHello;
+    use Acme\HelloBundle\DependencyInjection\UnconventionalExtensionClass;
 
     class AcmeHelloBundle extends Bundle
     {
@@ -513,7 +513,7 @@ your bundle::
             parent::build($container);
 
             // register extensions that do not follow the conventions manually
-            $container->registerExtension(new ExtensionHello());
+            $container->registerExtension(new UnconventionalExtensionClass());
         }
     }