Correct the type for challengeAnswer attribute. (#356)

Author: msailes

aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/CognitoUserPoolVerifyAuthChallengeResponseEvent.java

@@ -67,7 +67,7 @@ public static class Request extends CognitoUserPoolEvent.Request {
         /**
          * The answer from the user's response to the challenge.
          */
-        private Map<String, String> challengeAnswer;
+        private String challengeAnswer;
         /**
          * This boolean is populated when PreventUserExistenceErrors is set to ENABLED for your User Pool client
          */
@@ -76,7 +76,7 @@ public static class Request extends CognitoUserPoolEvent.Request {
         @Builder(setterPrefix = "with")
         public Request(Map<String, String> userAttributes,
                        Map<String, String> clientMetadata,
-                       Map<String, String> challengeAnswer,
+                       String challengeAnswer,
                        Map<String, String> privateChallengeParameters,
                        boolean userNotFound) {
             super(userAttributes);