minor symfony#12805 [Messenger] Added a missing class import (javiereguiluz)

This PR was merged into the 4.3 branch.

Discussion
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[Messenger] Added a missing class import

All examples in this article show all imports ... so this one was missing.

Commits
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d9fd6a7 [Messenger] Added a missing class import

Author: OskarStark

components/messenger.rst

@@ -163,6 +163,7 @@ Instead of dealing directly with the messages in the middleware you receive the
 Hence you can inspect the envelope content and its stamps, or add any::
 
     use App\Message\Stamp\AnotherStamp;
+    use Symfony\Component\Messenger\Envelope;
     use Symfony\Component\Messenger\Middleware\MiddlewareInterface;
     use Symfony\Component\Messenger\Middleware\StackInterface;
     use Symfony\Component\Messenger\Stamp\ReceivedStamp;