correct a typo in a namespace loading

the correct namespace is not `Symfony\Component\HttpKernel\DependencyInjection\Extension` but `Symfony\Component\DependencyInjection\Extension\Extension`.

The mistake is present on all versions of the doc I checked, so I propose the correction on the oldest LTS but ie the 4.2 also have the typo.


If using the typo, the code still works because `Symfony\Component\HttpKernel\DependencyInjection\Extension` extends `Symfony\Component\DependencyInjection\Extension\Extension`

Author: bruno-ds

bundles/extension.rst

@@ -35,8 +35,8 @@ but usually you would simply extend the
     // src/Acme/HelloBundle/DependencyInjection/AcmeHelloExtension.php
     namespace Acme\HelloBundle\DependencyInjection;
 
-    use Symfony\Component\HttpKernel\DependencyInjection\Extension;
     use Symfony\Component\DependencyInjection\ContainerBuilder;
+    use Symfony\Component\DependencyInjection\Extension\Extension;
 
     class AcmeHelloExtension extends Extension
     {