Fix favicon for dart-lang.github.io/dart_lint.

[pq]

Playing around with docs for rules over here:

http://dart-lang.github.io/dart_lint/lints/

and I can't seem to get the favicon to be picked up. I put it here

http://dart-lang.github.io/dart_lint/favicon.ico

thinking that's all I needed to do but no dice!

Surely something obvious...

[pq]

🤘