minor #20728 fix: Add missing ContainerBuilder use statement in Bundle documentation (jbonnier)

This PR was merged into the 6.4 branch.

Discussion
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fix: Add missing ContainerBuilder use statement in Bundle documentation

Add missing ContainerBuilder use statement in the "How to Load Service Configuration inside a Bundle" documentation

Commits
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a207201 fix: Add missing ContainerBuilder use statement in Bundle documentation

Author: OskarStark

bundles/extension.rst

@@ -29,6 +29,7 @@ class, you can define the :method:`Symfony\\Component\\HttpKernel\\Bundle\\Abstr
 method to load service definitions from configuration files::
 
     // ...
+    use Symfony\Component\DependencyInjection\ContainerBuilder;
     use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
     use Symfony\Component\HttpKernel\Bundle\AbstractBundle;