fix preview url

app/views/shared/_meta.html.erb

@@ -13,10 +13,10 @@
   <meta property="og:image" content="">
   <meta name="twitter:image" content="">
 <% else %>
-  <meta property="og:image" content="/preview.png">
+  <meta property="og:image" content="<%= root_url %>preview.png">
   <meta property="og:image:width" content="800">
   <meta property="og:image:height" content="800">
-  <meta name="twitter:image" content="/preview.png">
+  <meta name="twitter:image" content="<%= root_url %>preview.png">
 <% end %>
 <meta name="twitter:card" content="summary">
 <meta name="twitter:creator" content="@docusealco">