minor #20040 [Mercure] fix service definition example in testing section (dbu)

This PR was merged into the 5.4 branch.

Discussion
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[Mercure] fix service definition example in testing section

Tried to follow the example and noticed that this was wrong

Commits
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fa16f07 fix service definition example in testing section

Author: javiereguiluz

mercure.rst

@@ -673,8 +673,9 @@ sent:
 .. code-block:: yaml
 
     # config/services_test.yaml
-    mercure.hub.default:
-        class: App\Tests\Functional\Stub\HubStub
+    services:
+        mercure.hub.default:
+            class: App\Tests\Functional\Stub\HubStub
 
 As MercureBundle support multiple hubs, you may have to replace
 the other service definitions accordingly.