Meta: update the 404 to invite filing issues instead of email

Wording from https://github.com/whatwg/whatwg.org/blob/master/whatwg.org/404

404.html

@@ -17,7 +17,7 @@ <h1>Not Found</h1>
  unfortunately likely to break over time. You might be able to find
  what you want from <a href="/multipage/">the contents page</a>.</p>
  <p>If you have found a broken link on the WHATWG site itself, please
- e-mail <a href="mailto:ian@hixie.ch">Ian Hickson</a>. If you found a
- broken link from another site pointing to the WHATWG site, please
- let that site know of the problem instead. Thanks!</p>
+ <a href="https://github.com/whatwg/html/issues/new">file an issue</a>.
+ If you found a broken link from another site pointing to the WHATWG site,
+ please let that site know of the problem instead. Thanks!</p>
 </div>