getting started: highlight web app conffile as YAML

docs/getting-started.rst

@@ -214,6 +214,8 @@ You can now setup a local web app to browse what you have locally archived. The
 web app uses the configuration file ``~/.config/swh/webapp/webapp.yml``. Create
 it and fill it with something like::
 
+.. code-block:: yaml
+
   storage:
     cls: remote
     args: