Reword glossary entry for etcd (#14621)

* Link to official etcd documentation

* Reword etcd glossary entry

* Update etcd.md

content/en/docs/reference/glossary/etcd.md

@@ -15,5 +15,8 @@ tags:
 
 <!--more-->
 
-Always have a backup plan for etcd's data for your Kubernetes cluster. For in-depth information on etcd, see [etcd documentation](https://github.com/coreos/etcd/blob/master/Documentation/docs.md).
+If your Kubernetes cluster uses etcd as its backing store, make sure you have a
+[back up](/docs/tasks/administer-cluster/configure-upgrade-etcd/#backing-up-an-etcd-cluster) plan
+for those data.
 
+You can find in-depth information about etcd in the offical [documentation](https://etcd.io/docs/).