minor #10354 correct a typo in a namespace loading (bruno-ds)

This PR was merged into the 2.8 branch.

Discussion
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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`

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Commits
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b3ca081 correct a typo in a namespace loading

Author: javiereguiluz

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
     {