Fix relURL for custom_css (#252)

The way relURL was used meant that one needed to preface the custom_css
entries with a slash, or the href would be incorrectly set to
subdircss/mycss.css rather than subdir/css/mycss.css, in the case of
a baseURL in a subdir.

Co-authored-by: Nick White <git@njw.name>

layouts/_default/baseof.html

@@ -20,7 +20,7 @@
     {{ end }}
 
     {{ range .Site.Params.custom_css }}
-      <link rel="stylesheet" href="{{ relURL ($.Site.BaseURL) }}{{ . }}">
+      <link rel="stylesheet" href="{{ relURL (.) }}">
     {{ end }}
 
     {{ block "favicon" . }}