Update running-an-express-graphql-server.mdx

Author: Shubhdeep12

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

@@ -49,7 +49,7 @@ 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');