Rename CSS file for Primer compatibility

Author: parkerbxyz

_includes/head.html

@@ -3,8 +3,8 @@
   <link href="{{ site.baseurl }}/feed.xml" type="application/atom+xml" rel="alternate" title="{{ site.title }} Feed">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <link href="https://fonts.googleapis.com/css?family=Roboto:300,400" rel="stylesheet">
-  <link rel="stylesheet" href="{{ site.baseurl }}/assets/css/main.css">
-  
+  <link rel="stylesheet" href="{{ site.baseurl }}/assets/css/index.css">
+
   {% include seo.html %}
 
 </head>