wizard: add help text to rhel edge ref
wizard: add help text to rhel edge ref

The default ref for a rhel edge image is rhel/8/$ARCH/edge. The ref
field now states this.

jkozol/welder-web,weldr/welder-web,jkozol/welder-web,weldr/welder-web,jkozol/welder-web,weldr/welder-web,weldr/welder-web,jkozol/welder-web