Fix parent state ref

- For RepositoryState, its parent state is now
  defined as RepositoryState as well allowing
  to reference from a json files.
- Relates to #262

spring-statemachine-data/jpa/src/test/resources/data3.json

@@ -15,14 +15,14 @@
 		"@id": "3",
 		"_class": "org.springframework.statemachine.data.jpa.JpaRepositoryState",
 		"initial": true,
-		"parentState": "S2",
+		"parentState": "2",
 		"state": "S20"
 	},
 	{
 		"@id": "4",
 		"_class": "org.springframework.statemachine.data.jpa.JpaRepositoryState",
 		"initial": false,
-		"parentState": "S2",
+		"parentState": "2",
 		"state": "S21"
 	},
 	{