fix: link to GitHub for Hyperic Sigar (#1552)

sigar seems dormant: the website is not on 'https', and has become
entirely unreachable recently. The repo hasn't seen an update since
9 years.

Perhaps we should consider whether it's responsible to still link to
it at all - but for now let's at least link to the GitHub repo instead
of the defunct website.

Author: raboof

Diff for: docs/src/main/paradox/cluster-metrics.md

@@ -82,7 +82,7 @@ ClusterMetricsExtension.get(system).subscribe(metricsListenerActor);
 
 ## Hyperic Sigar Provisioning
 
-Both user-provided and built-in metrics collectors can optionally use [Hyperic Sigar](http://www.hyperic.com/products/sigar)
+Both user-provided and built-in metrics collectors can optionally use [Hyperic Sigar](https://github.com/hyperic/sigar)
 for a wider and more accurate range of metrics compared to what can be retrieved from ordinary JMX MBeans.
 
 Sigar is using a native o/s library, and requires library provisioning, i.e.