Documentation: change etcdctl role remove to etcdctl role delete

This is a document error. With running `etcdctl role --help`, we can find that it should be delete, not remove.

Fixes etcd-io#10849

Documentation/op-guide/authentication.md

@@ -119,7 +119,7 @@ $ etcdctl role revoke-permission myrolename /foo/bar
 As is removing a role entirely:
 
 ```
-$ etcdctl role remove myrolename
+$ etcdctl role delete myrolename
 ```
 
 ## Enabling authentication