[DOCS] Fix incorrect AD realm setting (#57520)

Author: Skoetting

docs/reference/settings/security-settings.asciidoc

@@ -578,7 +578,7 @@ this setting controls the amount of time to cache DNS lookups. Defaults
 to `1h`.
 
 `domain_name`::
-The domain name of Active Directory. If the `url` and the `user_search_dn`
+The domain name of Active Directory. If the `url` and the `user_search.base_dn`
 settings are not specified, the cluster can derive those values from this 
 setting. Required.