fix atom media type

_includes/head.html

@@ -22,5 +22,5 @@
   <link rel="shortcut icon" href="{{ site.baseurl }}public/favicon.ico">
 
   <!-- RSS -->
-  <link rel="alternate" type="application/rss+xml" title="RSS" href="{{ site.baseurl }}atom.xml">
+  <link rel="alternate" type="application/atom+xml" title="RSS" href="{{ site.baseurl }}atom.xml">
 </head>