[DOCS] deprecate kibana_user (#805)

Author: legrego

docs/en/stack/ml/anomaly-detection/getting-started.asciidoc

@@ -60,7 +60,7 @@ can do anything. For information about how to set that password, see
 If you are performing these steps in a production environment, take extra care
 because `elastic` has the `superuser` role and you could inadvertently make
 significant changes to the system. You can alternatively assign the
-`machine_learning_admin` and `kibana_user` roles to a user ID of your choice.
+`machine_learning_admin` and `kibana_admin` roles to a user ID of your choice.
 
 For more information, see <<built-in-roles>> and <<privileges-list-cluster>>.