Update Additional user-data example
Update Additional user-data example

Looking at https://godoc.org/k8s.io/kops/pkg/apis/kops#InstanceGroupSpec and https://github.com/kubernetes/kops/pull/3633, the field is `AdditionalUserData` not `ExtraUserData`
robinpercy/kops,andrewsykim/kops,chrislovecnm/kops,tsupertramp/kops,blakebarnett/kops,chrislovecnm/kops,mad01/kops,chrislovecnm/kops,andrewsykim/kops,pdh/kops,mad01/kops,mad01/kops,justinsb/kops,andrewsykim/kops,blakebarnett/kops,robinpercy/kops,kubernetes/kops,chrislovecnm/kops,robinpercy/kops,aledbf/kops,justinsb/kops,blakebarnett/kops,gambol99/kops,aledbf/kops,pdh/kops,pdh/kops,mad01/kops,aledbf/kops,tsupertramp/kops,gambol99/kops,tsupertramp/kops,justinsb/kops,kubernetes/kops,robinpercy/kops,aledbf/kops,pdh/kops,tsupertramp/kops,gambol99/kops,kubernetes/kops,blakebarnett/kops,gambol99/kops,andrewsykim/kops