Update docs/configuring-playbook-prometheus-grafana.md: add the note label

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-prometheus-grafana.md

@@ -34,7 +34,7 @@ Name | Description
 `prometheus_postgres_exporter_enabled`|[Postgres Exporter](configuring-playbook-prometheus-postgres.md) is an addon of sorts to expose Postgres database metrics to Prometheus.
 `matrix_prometheus_nginxlog_exporter_enabled`|[nginx Log Exporter](configuring-playbook-prometheus-nginxlog.md) is an addon of sorts to expose nginx logs to Prometheus.
 
-The retention policy of Prometheus metrics is [15 days by default](https://prometheus.io/docs/prometheus/latest/storage/#operational-aspects). Older data gets deleted automatically.
+**Note**: the retention policy of Prometheus metrics is [15 days by default](https://prometheus.io/docs/prometheus/latest/storage/#operational-aspects). Older data gets deleted automatically.
 
 ### Configure Grafana