Switch to using `icon` instead of `shortcut icon`.

Fixes HTML validation. Note that this didn't happen previously.

docs/_includes/header.html

@@ -4,7 +4,7 @@
 <meta name="description" content="{{ site.name }}: {{ site.description }}">
 <meta name="author" content="{{ site.authors }}">
 <meta name="viewport" content="initial-scale=1, maximum-scale=1, user-scalable=no, minimal-ui">
-<link rel="shortcut icon" href="/favicon.ico">
+<link rel="icon" href="/favicon.ico">
 <meta name="apple-mobile-web-app-capable" content="yes">
 <meta name="apple-mobile-web-app-status-bar-style" content="black">