added page title with react-helmet

Post.js.diff

 import React from 'react';
 import { createClient } from 'contentful';
+import Helmet from 'react-helmet';

     return (
       <React.Fragment>
+        <Helmet title={this.state.data.title} />
         <h1>{this.state.data.title}</h1>
         {this.state.data.content}
       </React.Fragment>
     );