Update dashboard_enabled on sample (#7316)

Since https://github.com/kubernetes-sigs/kubespray/pull/6804
dashboard_enabled has been false by default.
However we forgot to update it on sample inventory and it made
confusion.
This updates the sample inventory.

inventory/sample/group_vars/k8s-cluster/addons.yml

@@ -1,7 +1,7 @@
 ---
 # Kubernetes dashboard
 # RBAC required. see docs/getting-started.md for access details.
-# dashboard_enabled: true
+# dashboard_enabled: false
 
 # Helm deployment
 helm_enabled: false