Update 13/umbraco-commerce/how-to-guides/add-item

Co-authored-by: Dinh Tran <[email protected]>

Author: mr-shellychauhan

13/umbraco-commerce/how-to-guides/add-item

@@ -12,6 +12,7 @@ Create a new Document Type with template, we will call it Product Page with thes
 The Product Page template can be implemented like the below code.
 
 ```csharp
+@inherits Umbraco.Cms.Web.Common.Views.UmbracoViewPage<ProductPage>
 @{
 var store = Model.Value<StoreReadOnly>("store", fallback: Fallback.ToAncestors);
 var product = CommerceApi.Instance.GetProduct(store.Id, Model.Key.ToString(), "en-GB");