Prefer preview_class_name

This seems to be the proper way to get the class name of a preview

Author: koppen

demo/app/views/docs/components/show.html.erb

@@ -34,7 +34,7 @@
 
     <% if @introduction_preview %>
       <section>
-        <lookbook-embed preview="<%= @introduction_preview.preview_class %>" scenario="example"></lookbook-embed>
+        <lookbook-embed preview="<%= @introduction_preview.preview_class_name %>" scenario="example"></lookbook-embed>
       </section>
     <% end %>