Automatic merge from submit-queue

examples: fix kube-dns link

Currently `hack/update-munge-docs.sh` doesn't run cleanly on master:

```
/home/sjennings/projects/kubernetes/src/k8s.io/kubernetes/examples/cluster-dns/README.md
----
md-links:
On line 6: "../../build/kube-dns/": target not found

/home/sjennings/projects/kubernetes/src/k8s.io/kubernetes/examples/mysql-wordpress-pd/README.md
----
md-links:
On line 69: "../../build/kube-dns/": target not found

/home/sjennings/projects/kubernetes/src/k8s.io/kubernetes/examples/spark/README.md
----
md-links:
On line 26: "../../build/kube-dns/": target not found

FAIL: some manual changes are still required.
/home/sjennings/projects/kubernetes/src/k8s.io/kubernetes/examples/ requires manual changes.  See preceding errors.
```

This is due to an out of date link to kube-dns.  This PR fixes those links.