Update README.md for correct jenkins service account

[itewk]

When using the jenkins service account the add-role-to-user documentation still lists the "default" user when it should list the "jenkins" user.

[sabre1041]

@itewk I discovered an issue with the role being applied to the service account. I incorporated your correction into the change

https://github.com/sabre1041/ose-jenkins-cluster/commit/f259ead6f9820c95736427c2063670dc3cd7837f