fix tagged mapping value YAML syntax

Author: xabbuh

service_container/service_subscribers_locators.rst

@@ -256,10 +256,9 @@ argument of type ``service_locator``:
         # config/services.yaml
         services:
             App\CommandBus:
-                arguments:
-                    !service_locator
-                        App\FooCommand: '@app.command_handler.foo'
-                        App\BarCommand: '@app.command_handler.bar'
+                arguments: !service_locator
+                    App\FooCommand: '@app.command_handler.foo'
+                    App\BarCommand: '@app.command_handler.bar'
 
     .. code-block:: xml