minor #17270 Fix the code example leading to an error (qzminski)

This PR was squashed before being merged into the 6.1 branch.

Discussion
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Fix the code example leading to an error

Commits
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8bf5f9e Fix the code example leading to an error

Author: javiereguiluz

configuration/micro_kernel_trait.rst

@@ -274,7 +274,7 @@ add a service conditionally based on the ``foo`` value::
         public function loadExtension(array $config, ContainerConfigurator $container, ContainerBuilder $builder): void
         {
             if ($config['foo']) {
-                $container->set('foo_service', new \stdClass());
+                $builder->register('foo_service', \stdClass::class);
             }
         }
     }