includes head: fix favicon

Author: Claire Nord

_includes/head.html

@@ -2,7 +2,7 @@
     <meta charset="utf-8">
 
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
-    <link rel="icon" type="image/png" href="/favicon.png">
+    <link rel="icon" type="image/png" href="{{ site.baseurl }}/favicon.png">
 
     <meta name="twitter:card" content="summary_large_image">
     <meta name="og:url" content="{{ page.url | prepend: site.url }}">