Documentation/op-guide: Fix link to Prometheus 2.0 alerting rules

Documentation/op-guide/monitoring.md

@@ -100,7 +100,7 @@ Now Prometheus will scrape etcd metrics every 10 seconds.
 
 ### Alerting
 
-There is a set of default alerts for etcd v3 clusters for [Prometheus 1.x](./etcd3_alert.rules) as well as [Prometheus 2.x](./etcd3_alert.rules).
+There is a set of default alerts for etcd v3 clusters for [Prometheus 1.x](./etcd3_alert.rules) as well as [Prometheus 2.x](./etcd3_alert.rules.yml).
 
 > Note: `job` labels may need to be adjusted to fit a particular need. The rules were written to apply to a single cluster so it is recommended to choose labels unique to a cluster.