do not say user panel username comes from a file

docs/testing_instructions.md

@@ -26,10 +26,9 @@ functionality of your cluster and include the output in your feedback.
 Please open https://admin.oas.example.org in the browser. You should see
 "Welcome to OpenAppStack" and a Login button. Please try logging in.
 
-Your username is saved in
-`clusters/my-cluster/secrets/userbackend_admin_username` in your OpenAppStack
-directory on your local machine. The password is saved in
-`userbackend_admin_password` in the same directory.
+An admin user was generated with the username `admin`. The password is saved in
+`clusters/my-cluster/secrets/userbackend_admin_password` in the OpenAppStack
+directory on your local machine.
 
 After logging in to the user panel, please try to make a new user. Don't forget
 to give it a username and password, and press "Save" afterwards.