Use Spree::Variant#track_inventory translation

We actually delegate the value to the master variant
and most stores will already have the translation present.

(cherry picked from commit 32e41bf)

backend/app/views/spree/admin/products/_form.html.erb

@@ -146,7 +146,7 @@
           <%= f.field_container :track_inventory do %>
             <label>
               <%= f.check_box :track_inventory %>
-              <%= Spree::Product.human_attribute_name(:track_inventory) %>
+              <%= Spree::Variant.human_attribute_name(:track_inventory) %>
             </label>
           <% end %>
         </div>