Imported GraphQLString and fixed the error.

Author: suyashdas

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

@@ -52,27 +52,32 @@ console.log('Running a GraphQL API server at http://localhost:4000/graphql');
 const { GraphQLObjectType, GraphQLSchema, GraphQLString } = require('graphql');
 const { createHandler } = require('graphql-http/lib/use/express');
 const express = require('express');
- 
-// Construct a schema and resolver
+
+// Construct a schema
 const schema = new GraphQLSchema({
   query: new GraphQLObjectType({
     name: 'Query',
     fields: {
-      hello: { 
-        type: GraphQLString,
-        resolve: () => 'Hello world!'
-      },
+      hello: { type: GraphQLString },
     },
   }),
 });
- 
+
+// 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
+    schema: schema,
+    rootValue: root,
   }),
 );