Fix heading level for blackbox exporter

with the move I should've fixed that

howto/prometheus.md

@@ -1421,7 +1421,7 @@ would otherwise be around long enough for Prometheus to scrape their
 metrics. We use it as a workaround to bridge Metrics data with
 Prometheus/Grafana.
 
-### Blackbox exporter
+## Blackbox exporter
 
 Most exporters are pretty straightforward: a service binds to a port and exposes
 metrics through HTTP requests on that port, generally on the `/metrics` URL.