Imported GraphQLString and fixed the resolver part as per code first approach.

Author: suyashdas

website/pages/docs/running-an-express-graphql-server.mdx

@@ -49,35 +49,30 @@ console.log('Running a GraphQL API server at http://localhost:4000/graphql');
 </Tabs.Tab>
 <Tabs.Tab>
 ```javascript
-const { GraphQLObjectType, GraphQLSchema } = require('graphql');
+const { GraphQLObjectType, GraphQLSchema, GraphQLString } = require('graphql');
 const { createHandler } = require('graphql-http/lib/use/express');
 const express = require('express');
-
-// Construct a schema
+ 
+// Construct a schema and resolver
 const schema = new GraphQLSchema({
   query: new GraphQLObjectType({
     name: 'Query',
     fields: {
-      hello: { type: GraphQLString },
+      hello: { 
+        type: GraphQLString,
+        resolve: () => 'Hello world!'
+      },
     },
   }),
 });
-
-// The root provides a resolver function for each API endpoint
-const root = {
-  hello() {
-    return 'Hello world!';
-  },
-};
-
+ 
 const app = express();
-
+ 
 // Create and use the GraphQL handler.
 app.all(
   '/graphql',
   createHandler({
-    schema: schema,
-    rootValue: root,
+    schema: schema
   }),
 );