Use a non-zero duration for group_interval (#12)

Alertmanager rejects zero: <https://github.com/prometheus/alertmanager/blob/v0.24.0/config/config.go#L786>

Author: greed42

README.md

@@ -142,8 +142,10 @@ route:
       - match:
           severity: heartbeat
         receiver: prommsd
-        # Always set group_interval to 0s, prommsd does its own grouping.
-        group_interval: 0s
+        # Always set group_interval to a very short non-zero
+        # duration: prommsd does its own grouping and alertmanager
+        # requires non-zero.
+        group_interval: 1s
         repeat_interval: 1m
 ```