minor #14344 Missing Namespace from event dispatcher (TheGarious)

This PR was submitted for the master branch but it was merged into the 4.4 branch instead.

Discussion
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Missing Namespace from event dispatcher

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Commits
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1e202b8 Missing Namespace

Author: OskarStark

event_dispatcher.rst

@@ -289,6 +289,8 @@ This alias mapping can be extended for custom events by registering the
 compiler pass ``AddEventAliasesPass``::
 
     // src/Kernel.php
+    namespace App;
+
     use App\Event\MyCustomEvent;
     use Symfony\Component\DependencyInjection\ContainerBuilder;
     use Symfony\Component\EventDispatcher\DependencyInjection\AddEventAliasesPass;