At some point there was an update to this site and many links were changes. For example, the url you posted is now at https://www.advancedcustomfields.com/resources/acf-init/. It appears that at one time the / in hook and filter names was removed and now it’s converted to a -. These types of errors should be reported to the developer by submitting a new support ticket here https://support.advancedcustomfields.com/new-ticket/
