Recommend using root controller attribute in route definition

Instead of `defaults._controller`

Author: andrew-demb

controller/service.rst

@@ -110,7 +110,7 @@ which is a common practice when following the `ADR pattern`_
         # config/routes.yaml
         hello:
             path:     /hello/{name}
-            defaults: { _controller: app.hello_controller }
+            controller: app.hello_controller
 
     .. code-block:: xml