simplify favicon path

Author: cremno

_includes/header.html

@@ -18,4 +18,4 @@
         <link rel="stylesheet" href="/css/main.css">
 
         <!-- Favicon -->
-        <link rel="icon" type="image/png" href="http://www.mruby.org/favicon.png" />
+        <link rel="icon" type="image/png" href="/favicon.png" />