add a copy of the sample image to the theme

layouts/partials/site-header.html

@@ -1,5 +1,5 @@
 {{ if .Params.featured_image }}
-  <header class="cover bg-center" style="background-image: url('{{ .Params.featured_image  | absURL }}');">
+  <header class="cover bg-center" style="background-image: url('{{ .Params.featured_image | absURL }}');">
     <div class="bg-black-60">
       {{ partial "site-navigation.html" .}}
       <div class="tc-l pv4 pv6-l ph3 ph4-ns">