Change metric names in release notes too

(cherry picked from commit 1ea83be)

Author: deadtrickster

release-notes/3.9.4.md

@@ -17,8 +17,8 @@ consistent release schedule.
 
  * New Prometheus metrics for alarms:
    * `rabbitmq_alarms_file_descriptor_limit` 1|0
-   * `rabbitmq_alarms_disk_limit` 1|0
-   * `rabbitmq_alarms_memory_limit` 1|0
+   * `rabbitmq_alarms_free_disk_space_watermark` 1|0
+   * `rabbitmq_alarms_memory_used_watermark` 1|0
 
    While some of the alarms have cluster-wide effect, these metrics are node-local.