fix url to prometheus architecture diagram

howto/prometheus.md

@@ -792,7 +792,7 @@ basic architecture of a Prometheus site:
 
 [Prometheus overview documentation]: https://prometheus.io/docs/introduction/overview/
 
-<img src="https://raw.githubusercontent.com/prometheus/alertmanager/master/doc/arch.svg" alt="A
+<img src="https://prometheus.io/assets/architecture.png" alt="A
 drawing of Prometheus' architecture, showing the push gateway and
 exporters adding metrics, service discovery through file_sd and
 Kubernetes, alerts pushed to the Alertmanager and the various UIs