Update site footer from 2017 -> 2018

Summary:
**Summary**

Update site footer from 2017 -> 2018
Closes #1592

Differential Revision: D6885937

fbshipit-source-id: d4077b49b363ace6ac89d57155ff909a25c49eef

Author: puopg

website/core/Site.js

@@ -50,7 +50,7 @@ var Site = React.createClass({
             {this.props.children}
 
             <footer className="wrap">
-              <div className="right">&copy; 2017 Facebook Inc.</div>
+              <div className="right">&copy; 2018 Facebook Inc.</div>
             </footer>
           </div>