minor #13088 Finish #12722 "Update micro_kernel_trait.rst" (amine-betari, wouterj)

This PR was merged into the 3.4 branch.

Discussion
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Finish #12722 "Update micro_kernel_trait.rst"

Closes #12722

Commits
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f3fd946 [#12722] Added filename comment to avoid confusion
b96feb6 Update micro_kernel_trait.rst

Author: wouterj

configuration/micro_kernel_trait.rst

@@ -23,6 +23,7 @@ via Composer:
 
 Next, create an ``index.php`` file that creates a kernel class and executes it::
 
+    // index.php
     use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
     use Symfony\Component\Config\Loader\LoaderInterface;
     use Symfony\Component\DependencyInjection\ContainerBuilder;