update link to documentation (grafana#99)

Author: tonglil

website/docs/r/alert_notification.html.md

@@ -36,7 +36,7 @@ The following arguments are supported:
 * `is_default` - (Optional) Is this the default channel for all your alerts.
 * `send_reminder` - (Optional) Whether to send reminders for triggered alerts.
 * `frequency` - (Optional) Frequency of alert reminders. Frequency must be set if reminders are enabled.
-* `settings` - (Optional) Additional settings, for full reference lookup [Grafana HTTP API documentation](http://docs.grafana.org/http_api/alerting).
+* `settings` - (Optional) Additional settings, for full reference lookup [Grafana HTTP API documentation](https://grafana.com/docs/grafana/latest/http_api/alerting_notification_channels/).
 
 **Note:** In `settings` the strings `"true"` and `"false"` are mapped to boolean `true` and `false` when sent to Grafana.