Update examples/vars.yml: update a comment to sync with roles/custom/matrix-base/defaults/main.yml

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

examples/vars.yml

@@ -3,7 +3,7 @@
 # Matrix user ids for your server will be of the form (`@user:example.com`).
 #
 # Note: this playbook does not touch the server referenced here.
-# Installation happens on another server ("matrix.example.com").
+# Installation happens on another server ("matrix.example.com", see `matrix_server_fqn_matrix`).
 #
 # If you've deployed using the wrong domain, you'll have to run the Uninstalling step,
 # because you can't change the Domain after deployment.