Fix invalid sample code (#3951)

Author: sakesun

website/docs/tutorials/index.md

@@ -18,25 +18,25 @@ npm install graphql --save
 To handle GraphQL queries, we need a schema that defines the `Query` type, and we need an API root with a function called a “resolver” for each API endpoint. For an API that just returns “Hello world!”, we can put this code in a file named `server.js`:
 
 ```js
-var { graphql, buildSchema } = require('graphql');
+let { graphql, buildSchema } = require('graphql');
 
 // Construct a schema, using GraphQL schema language
-var schema = buildSchema(`
+let schema = buildSchema(`
   type Query {
     hello: String
   }
 `);
 
 // The root provides a resolver function for each API endpoint
-var root = {
+let root = {
   hello: () => {
     return 'Hello world!';
   },
 };
 
 // Run the GraphQL query '{ hello }' and print out the response
-graphql(schema, '{ hello }', root).then((response) => {
-  console.log(response);
+graphql({ schema, source: '{ hello }', rootValue: root }).then((response) => {
+  console.log(JSON.stringify(response, null, 2));
 });
 ```
 
@@ -50,8 +50,8 @@ You should see the GraphQL response printed out:
 
 ```js
 {
-  data: {
-    hello: 'Hello world!';
+  "data": {
+    "hello": "Hello world!"
   }
 }
 ```