Correct URL of the favicon in the documentation

img/favicon.png does not seem to exist,
so follow the example in docs/layouts/index.html
and use /favicon.ico instead.

docs/layouts/partials/header.html

@@ -6,7 +6,7 @@
     <meta name="description" content="">
     <meta name="author" content="">
     <meta name="keyword" content="">
-    <link rel="shortcut icon" href="img/favicon.png">
+    <link rel="shortcut icon" href="/favicon.ico" type="image/x-icon" />
 
     <title>{{.Title}}</title>