kubeadm: add corednsdeployment as a patch target

Author: SataQiu

content/en/docs/setup/production-environment/tools/kubeadm/control-plane-flags.md

@@ -168,8 +168,7 @@ patches:
 The directory must contain files named `target[suffix][+patchtype].extension`.
 For example, `kube-apiserver0+merge.yaml` or just `etcd.json`.
 
-- `target` can be one of `kube-apiserver`, `kube-controller-manager`, `kube-scheduler`, `etcd`
-and `kubeletconfiguration`.
+- `target` can be one of `kube-apiserver`, `kube-controller-manager`, `kube-scheduler`, `etcd`, `kubeletconfiguration` and `corednsdeployment`.
 - `patchtype` can be one of `strategic`, `merge` or `json` and these must match the patching formats
 [supported by kubectl](/docs/tasks/manage-kubernetes-objects/update-api-object-kubectl-patch).
 The default `patchtype` is `strategic`.