Add missing RST directive

Author: javiereguiluz

src/TwigComponent/doc/index.rst

@@ -369,6 +369,8 @@ The only limitation when defining contents inside a component using the HTML syn
 is that you cannot import macros using the ``_self`` keyword. You must always use
 the full template path:
 
+.. code-block:: html+twig
+
     <twig:Alert>
         {# ❌ this won't work #}
         {% from _self import message_formatter %}