Update slack channel for gitaly alerts

alerts/gitaly.rules

@@ -4,7 +4,7 @@ gitaly:grpc_server_handled_total:error_max_rate1m = max(gitaly:grpc_server_handl
 ALERT gitaly_error_rate_too_high
   IF gitaly:grpc_server_handled_total:error_max_rate1m > 5
   FOR 5m
-  LABELS {severity="critical", channel="gitaly-alerts"}
+  LABELS {severity="critical", channel="gitaly"}
   ANNOTATIONS {
     title="Gitaly error rate is too high: {{$value | printf \"%.2f\" }}",
     description="Gitaly error rate for the last 20 minutes is over 5. Check Gitaly logs and consider disabling it.",