contrib/cni: Fix README link
The CNI plugins are now living in a repo of their own.
@@ -13,4 +13,4 @@ In addition, you need to install the [CNI plugins][cni] necessary into
two plugins necessary for the example CNI configurations are `loopback` and
`bridge`.
-[cni]: https://github.com/containernetworking/cni
+[cni]: https://github.com/containernetworking/plugins
