Update mercure.rst

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

Author: javiereguiluz

mercure.rst

@@ -318,7 +318,7 @@ Install it, then use it through the ``AbstractController::addLink`` helper metho
 Then, this header can be parsed client-side to find the URL of the Hub,
 and to subscribe to it:
 
-.. code-block:: html
+.. code-block:: javascript
 
     <script>
     // Fetch the original resource served by the Symfony web API