Fix atom.xml namespace

Author: josevalim

atom.xml

@@ -3,7 +3,7 @@ layout: null
 elixir_url: https://elixir-lang.org
 ---
 <?xml version="1.0" encoding="utf-8"?>
-<feed xmlns="https://www.w3.org/2005/Atom">
+<feed xmlns="http://www.w3.org/2005/Atom">
 
 <title>Elixir Lang</title>
 <link href="{{ page.elixir_url }}/atom.xml" rel="self"/>