Rename RSSlink to RSSLink

The former will be deprecated and eventually removed from Hugo.

Note: Currently both of them exist in Hugo, which is the reason for the cleanup.

layouts/partials/head.html

   <link rel="shortcut icon" href="/favicon.png">
 
   <!-- RSS -->
-  <link href="{{ .RSSlink }}" rel="alternate" type="application/rss+xml" title="{{ .Site.Title }}" />
+  <link href="{{ .RSSLink }}" rel="alternate" type="application/rss+xml" title="{{ .Site.Title }}" />
 </head>