Adding kube-proxy to this list

This list appears to name binaries whose configuration flags are documented in this section: [Command Line Tools Reference](/docs/reference/command-line-tools-reference/). Since kube-proxy is an item in that reference, it seems helpful to give it an entry and link here.

Author: JayBeale

content/en/docs/concepts/extend-kubernetes/_index.md

@@ -39,6 +39,7 @@ Customization approaches can be broadly divided into *configuration*, which only
 *Configuration files* and *flags* are documented in the Reference section of the online documentation, under each binary:
 
 * [kubelet](/docs/reference/command-line-tools-reference/kubelet/)
+* [kube-proxy](/docs/reference/command-line-tools-reference/kube-proxy/)
 * [kube-apiserver](/docs/reference/command-line-tools-reference/kube-apiserver/)
 * [kube-controller-manager](/docs/reference/command-line-tools-reference/kube-controller-manager/)
 * [kube-scheduler](/docs/reference/command-line-tools-reference/kube-scheduler/).