message: docs: point user to add user_data instead of nickname directly
diff: @@ -165,7 +165,7 @@ Here is an example on the way to proceed to add a `nickname` value to the regist
  1. Add a column `nickname` of type text to the `public.users` table
  2. Set the environment variable `REGISTRATION_CUSTOM_FIELDS=nickname`
  -3. The registration endpoint now expects a `nickname` value in addition to `email` and `password`
  +3. The registration endpoint now expects a `user_data` value in addition to `email` and `password` that has `{"nickname": "Some Nickname"}`
  ::: warning
  Any given field must exist in the `users` GraphQL type that corresponds to the `public.users` PostgreSQL table, or registration will fail.
  