fix(docs): update extend repository (#6140)

- Issue ticket https://github.com/medusajs/medusa/issues/6139
- Update document how to extend repository

www/apps/docs/content/development/entities/extend-repository.md

@@ -54,7 +54,7 @@ export const ProductRepository = dataSource
   .extend({
     // it is important to spread the existing repository here.
     //  Otherwise you will end up losing core properties
-    ...MedusaProductRepository,
+    ...Object.assign(MedusaProductRepository, { target: Product }),
 
     /**
      * Here you can create your custom function