Modify scheduled publication date validation error message

The updated message is closer to the guidance provided at https://design-system.service.gov.uk/components/date-input/, but the guidance doesn't cover cases where the user enters, for example, a string instead of a number for one of the date parts. It also assumes that invidividual parts of the date can be validated separately. For this reason I have had to improvise a little, but I believe the new message is closer to the spirit of the guidance and does not feature the unapproved XX style formatting advice.

app/models/edition.rb

@@ -736,7 +736,7 @@ def scheduled_publication=(date)
 
   def valid_date
     if @date_field_validity.present? && @date_field_validity[:scheduled_publication] == false
-      errors.add(:scheduled_publication, "must be a valid date in the format XX XX XXXX")
+      errors.add(:scheduled_publication, "must be a valid date in the correct format")
     end
   end