doc(example): update api_backend example to use ES6 arrow function immediate return (aws#446)

Since this code is using ES6 we can use the short version of returning a param

Author: SamanShafigh

examples/2016-10-31/api_backend/src/index.js

@@ -6,13 +6,7 @@ const dynamo = new AWS.DynamoDB.DocumentClient();
 
 const tableName = process.env.TABLE_NAME;
 
-const createResponse = (statusCode, body) => {
-    
-    return {
-        statusCode: statusCode,
-        body: body
-    }
-};
+const createResponse = (statusCode, body) => ({ statusCode, body });
 
 exports.get = (event, context, callback) => {