Update mercure.rst

Co-Authored-By: dunglas <[email protected]>

Author: OskarStark

mercure.rst

@@ -333,7 +333,7 @@ Authorization
 
 Mercure also allows to dispatch updates only to authorized clients.
 To do so, set the list of **targets** allowed to receive the update
-as the third parameter of the ``Update`` constructor:
+as the third parameter of the ``Update`` constructor::
 
 .. code-block:: php