Please note that this is not required if you have build GVM-20.08 as this release already includes the sync script pointing at the updated feed server.
If you had to change the URL you probably have some older sync scripts in your PATH which takes precedence over the updated ones from GVM-20.08.
I would strongly suggest to check how you have build your setup and to remove remains / leftovers from your old installation (at least the sync scripts are still the ones from an older GVM installation if you had to change the URL).
If the suggestion isn’t followed you might get into trouble into the future due to using outdated sync scripts not getting any updates.