Update kubeadm-upgrade-1-9.md (#7338)

If we are getting releases from the stable channel, I think the plan output would say stable instead of experimental. At least, that is what I see in this video, around 40:15 mins in: https://www.youtube.com/watch?v=x9doB5eJWgQ&feature=youtu.be

docs/tasks/administer-cluster/kubeadm-upgrade-1-9.md

@@ -102,7 +102,7 @@ Components that must be upgraded manually after you've upgraded the control plan
 COMPONENT   CURRENT      AVAILABLE
 Kubelet     1 x v1.8.1   v1.9.0
 
-Upgrade to the latest experimental version:
+Upgrade to the latest stable version:
 
 COMPONENT            CURRENT   AVAILABLE
 API Server           v1.8.1    v1.9.0