minor symfony#20339 [Messenger] Update external link on doc (MrYamous)

This PR was merged into the 5.4 branch.

Discussion
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[Messenger] Update external link on doc

According to message on [currently linked site](https://www.freedesktop.org/wiki/Software/systemd/) I suggest to replace it with the new one dedicated to systemd

Commits
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9c81e1c update external link on messenger doc

Author: javiereguiluz

messenger.rst

@@ -2695,7 +2695,7 @@ Learn more
 .. _`streams`: https://redis.io/topics/streams-intro
 .. _`Supervisor docs`: http://supervisord.org/
 .. _`PCNTL`: https://www.php.net/manual/book.pcntl.php
-.. _`systemd docs`: https://www.freedesktop.org/wiki/Software/systemd/
+.. _`systemd docs`: https://systemd.io/
 .. _`SymfonyCasts' message serializer tutorial`: https://symfonycasts.com/screencast/messenger/transport-serializer
 .. _`Long polling`: https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-short-and-long-polling.html
 .. _`Visibility Timeout`: https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-visibility-timeout.html