[Messenger] Fix some syntax issue

Author: javiereguiluz

messenger.rst

@@ -2052,7 +2052,7 @@ The transport has a number of options:
 
 .. versionadded:: 7.2
 
-    Keepalive support, using the `--keepalive` option, was added in Symfony 7.2.
+    Keepalive support, using the ``--keepalive`` option, was added in Symfony 7.2.
 
 Serializing Messages
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