[DOCS] Fixed cross doc xref in plugin docs.

docs/plugins/repository-gcs.asciidoc

@@ -92,7 +92,7 @@ A service account file looks like this:
 ----
 // NOTCONSOLE
 
-This file must be stored in the <<secure-settings, elasticsearch keystore>>, under a setting name
+This file must be stored in the {ref}/secure-settings.html[elasticsearch keystore], under a setting name
 of the form `gcs.client.NAME.credentials_file`, where `NAME` is the name of the client congiguration.
 The default client name is `default`, but a different client name can be specified in repository
 settings using `client`.