minor #19721 [HttpKernel] Update micro_kernel_trait.rst (featuriz)

This PR was submitted for the 7.0 branch but it was merged into the 6.4 branch instead.

Discussion
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[HttpKernel] Update micro_kernel_trait.rst

ContainerBuilder class is missing in src/Kernel.php

`use Symfony\Component\DependencyInjection\ContainerBuilder;`

Commits
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be86770 Update micro_kernel_trait.rst

Author: javiereguiluz

configuration/micro_kernel_trait.rst

@@ -230,6 +230,7 @@ Now it looks like this::
 
     use App\DependencyInjection\AppExtension;
     use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
+    use Symfony\Component\DependencyInjection\ContainerBuilder;
     use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
     use Symfony\Component\HttpKernel\Kernel as BaseKernel;
     use Symfony\Component\Routing\Loader\Configurator\RoutingConfigurator;