minor #15282 Recommend using root controller attribute in route definition (andrew-demb)

This PR was merged into the 4.4 branch.

Discussion
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Recommend using root `controller` attribute in route definition

Also, it is consistent with the example above:
```
# config/routes.yaml
hello:
    path:     /hello
    controller: App\Controller\HelloController::index
    methods: GET
```

Checked on Symfony 5.2.6

Commits
-------

1a7717a Recommend using root `controller` attribute in route definition

Author: javiereguiluz

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