fix: whitelist doc method

This should've been whitelisted, looks like it was missed out

closes https://github.com/frappe/erpnext/issues/34898

(cherry picked from commit e4f152a416)

erpnext/e_commerce/doctype/website_item/website_item.py

@@ -315,6 +315,7 @@ class WebsiteItem(WebsiteGenerator):
 			self.item_code, skip_quotation_creation=True
 		)
 
+	@frappe.whitelist()
 	def copy_specification_from_item_group(self):
 		self.set("website_specifications", [])
 		if self.item_group: