fix doc link for addons
* [SSH requirements](ssh.md)
* [Upgrading Kubernetes Clusters Using KubeOne](upgrading_cluster.md)
-* [Using Addons](adding_provider_support.md)
+* [Using Addons](addons.md)
* [Proxy support](proxy.md)
* [Manual Cluster Repair](manual-cluster-repair.md)
* [Environment variables used by KubeOne](environment_variables.md)
