Change digital ocean autoscale to use stable coreos channel
What does this MR do?
When I was setting up a new Gitlab runner infrastructure, I had errors using CoreOS Beta channel. I confirmed that it was happening with vanilla Docker Machine and filed a bug here: https://gitlab.com/gitlab-org/gitlab-ce/issues/26100#note_20582668
This tiny merge request changes the docs to use the CoreOS stable channel which seemed to work fine for me.