fixing links on website

Summary:
fixing broken links for:
- Twitter account
- App download
- FlipperKit image on landing page

Reviewed By: passy

Differential Revision: D9131794

fbshipit-source-id: 0e7db0ebd57757684571f0325543c35ce3ba9bec

website/core/Footer.js

@@ -47,7 +47,7 @@ class Footer extends React.Component {
           </div>
           <div>
             <h5>More</h5>
-            <a href="https://twitter.com/fb_flipper">Twitter</a>
+            <a href="https://twitter.com/flipper_fb">Twitter</a>
             <a href={this.props.config.repoUrl}>GitHub</a>
             <a
               className="github-button"