minor #19226 Add missing use statements (npotier)

This PR was submitted for the 7.0 branch but it was merged into the 5.4 branch instead.

Discussion
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Add missing use statements

Adding the two required namespaces in the code sample

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Commits
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dc5e8e7 Update multiple_kernels.rst

Author: OskarStark

configuration/multiple_kernels.rst

@@ -117,6 +117,9 @@ resources::
     // src/Kernel.php
     namespace Shared;
 
+    // ...
+    use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
+    use Symfony\Component\Routing\Loader\Configurator\RoutingConfigurator;
     // ...
 
     class Kernel extends BaseKernel