fix admin display of digital products

copied from funkensturm's fix.

app/views/spree/admin/digitals/index.html.erb

@@ -10,9 +10,9 @@
 <% else %>
   This product has no variants.
 
-  <% if @product.master.digital? %>
+ <% if @product.master.digital? %>
     A digital version of this product currently exists:
-    <%= render @product.master.digital %>
+    <%= render @product.master.digitals %>
   <% end %>
 
   <%= render 'form', :variant => @product.master %>