Add "type" to stylesheet link tag.

docs/_includes/head.html

@@ -17,4 +17,4 @@
 
 <link href="vendor/css/font-awesome.min.css" type="text/css" rel="stylesheet" />
 <link href="vendor/css/anchor.css" type="text/css" rel="stylesheet" />
-<link href="css/s2-docs.css" rel="stylesheet" >
+<link href="css/s2-docs.css" type="text/css" rel="stylesheet" >