minor #15626 [Mercure] Update mercure.rst Symfony Local Web Server url (cybervoid)

This PR was submitted for the 5.3 branch but it was merged into the 4.4 branch instead.

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[Mercure] Update mercure.rst Symfony Local Web Server url

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9189863 Update mercure.rst Symfony Local Web Server url

Author: javiereguiluz

mercure.rst

@@ -66,7 +66,7 @@ clients.
 
 .. image:: /_images/mercure/schema.png
 
-If you use the `Symfony Local Web Server </setup/symfony_server>`_ or `Symfony Docker`_,
+If you use the `Symfony Local Web Server </doc/current/setup/symfony_server>`_ or `Symfony Docker`_,
 a Mercure Hub is automatically available.
 
 For production usage, an official and open source (AGPL) Hub based on the Caddy web server