minor: Turn HTML comment into template comment.

We don't need to send comments about our portico
setup over the wire.

templates/zerver/base.html

@@ -28,9 +28,9 @@
         {% block customhead %}
         {% endblock %}
 
-        <!-- this is required because we want to put a custom head in
+        {# this is required because we want to put a custom head in
         `zerver/portico.html` that isn't overwritten like the
-        `customhead` -->
+        `customhead` #}
         {% block porticocustomhead %}
         {% endblock %}
     </head>