docs(appsync): fix argument in Values.projecting() call (#12855)

Customer reported incorrect argument to Values.projecting() in appsync README

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Jerry Kindall

packages/@aws-cdk/aws-appsync/README.md

@@ -89,7 +89,7 @@ demoDS.createResolver({
   fieldName: 'addDemo',
   requestMappingTemplate: appsync.MappingTemplate.dynamoDbPutItem(
     appsync.PrimaryKey.partition('id').auto(),
-    appsync.Values.projecting('demo')
+    appsync.Values.projecting('input')
   ),
   responseMappingTemplate: appsync.MappingTemplate.dynamoDbResultItem(),
 });