update federation control plane turn down command (#7332)

**kubectl** uses `--context` option for specifying the context, the option `--host-cluster-context` is for **kubefed join**.

docs/tasks/federation/set-up-cluster-federation-kubefed.md

@@ -483,7 +483,7 @@ federation control plane's etcd. You can delete the federation
 namespace by running the following command:
 
 ```
-kubectl delete ns federation-system --host-cluster-context=rivendell
+kubectl delete ns federation-system --context=rivendell
 ```
 
 Note that `rivendell` is the host cluster name, replace that with the