Fix `'something' is undefined` error in matrix-bridge-appservice-slack

.. when `matrix_nginx_proxy_enabled: false`

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2177

roles/matrix-bridge-appservice-slack/tasks/init.yml

@@ -85,7 +85,7 @@
     msg: >-
       NOTE: You've enabled the Matrix Slack bridge but are not using the matrix-nginx-proxy
       reverse proxy.
-      Please make sure that you're proxying the `{{ something }}`
+      Please make sure that you're proxying the `{{ matrix_appservice_slack_public_endpoint }}`
       URL endpoint to the matrix-appservice-slack container.
       You can expose the container's port using the `matrix_appservice_slack_container_http_host_bind_port` variable.
   when: "matrix_appservice_slack_enabled | bool and not matrix_nginx_proxy_enabled | default(False) | bool"