Fix web manifest for sub-dirs

See #312

layouts/partials/header.html

@@ -84,7 +84,7 @@
   <link rel="feed" href="{{ .RSSLink | default .Site.RSSLink }}" type="application/rss+xml" title="{{ .Site.Title }}">
   {{ end }}
 
-  <link rel="manifest" href="site.webmanifest">
+  <link rel="manifest" href="{{ "site.webmanifest" | relURL }}">
   <link rel="icon" type="image/png" href="{{ "/img/icon.png" | relURL }}">
   <link rel="apple-touch-icon" type="image/png" href="{{ "/img/icon-192.png" | relURL }}">