build: fix schema generation (#821)

Author: Dane Pilcher

packages/appsync-modelgen-plugin/src/schemas/introspection/1/ModelIntrospectionSchema.json renamed to packages/appsync-modelgen-plugin/schemas/introspection/1/ModelIntrospectionSchema.json

@@ -275,7 +275,8 @@
             },
             "required": [
                 "associatedWith",
-                "connectionType"
+                "connectionType",
+                "targetNames"
             ]
         },
         "AssociationBelongsTo": {