Update link of PostgreSQL Server Exporter in postgres_exporter.md

This repository has been integrated into the [Prometheus Monitoring Community  organization](https://github.com/prometheus-community),
so the repository URL has changed to `https://github.com/prometheus-community/postgres_exporter`.
GitHub is automatically redirecting, this change makes sure the docs reflect the move so attentive users aren't surprised by mismatching names.

doc/administration/monitoring/prometheus/postgres_exporter.md

@@ -6,7 +6,7 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 
 # PostgreSQL Server Exporter **(FREE SELF)**
 
-The [PostgreSQL Server Exporter](https://github.com/wrouesnel/postgres_exporter) allows you to export various PostgreSQL metrics.
+The [PostgreSQL Server Exporter](https://github.com/prometheus-community/postgres_exporter) allows you to export various PostgreSQL metrics.
 
 For installations from source you must install and configure it yourself.