Update deprecated link in doc (#2607)

Currently, the modified link is out of date. This patch aims to update
this link to the working one that user can access.
Co-Authored-By: Nguyen Phuong An <AnNP@vn.fujitsu.com>
Signed-off-by: Kim Bao Long <longkb@vn.fujitsu.com>

plugin/kubernetes/README.md

@@ -13,7 +13,7 @@ CoreDNS running the kubernetes plugin can be used as a replacement for kube-dns
 cluster.  See the [deployment](https://github.com/coredns/deployment) repository for details on [how
 to deploy CoreDNS in Kubernetes](https://github.com/coredns/deployment/tree/master/kubernetes).
 
-[stubDomains and upstreamNameservers](https://blog.kubernetes.io/2017/04/configuring-private-dns-zones-upstream-nameservers-kubernetes.html)
+[stubDomains and upstreamNameservers](https://kubernetes.io/blog/2017/04/configuring-private-dns-zones-upstream-nameservers-kubernetes/)
 are implemented via the *proxy* plugin and kubernetes *upstream*. See example below.
 
 This plugin can only be used once per Server Block.