Merge pull request #28871 from tengqm/kubeadm-config-1.21

Update kubeadm config reference for 1.21

Author: k8s-ci-robot

content/en/docs/reference/_index.md

@@ -81,7 +81,7 @@ operator to use or manage a cluster.
 
 ## Config API for kubeadm
 
-* [v1beta3](/docs/reference/config-api/kubeadm-config.v1beta3/)
+* [v1beta2](/docs/reference/config-api/kubeadm-config.v1beta2/)
 
 ## Design Docs