Fixes link in help text for adding credentials

The hardcoded '../../credentials' pointed to a wrong location if you have
Jenkins running e.g. 'on www.servername.com/ci'. The 'rootURL' handles
this.

Change-Id: I8b6d529aca31b5b7a28d02fde0c4b86b9dbb7b3c
Signed-off-by: Aske Olsson <aske.olsson@switch-gears.dk>