exporters.md: Point to non-stale PowerDNS exporter (#725)
exporters.md: Point to non-stale PowerDNS exporter (#725)

The fork is working, the original one does not even compile any more.
du2016/prometheus-docs,ddfreyne/prometheus-docs,chosenken/docs,prometheus/docs,prometheus/docs,chosenken/docs,prometheus/docs,du2016/prometheus-docs,chosenken/docs,du2016/prometheus-docs,chosenken/docs,ddfreyne/prometheus-docs,ddfreyne/prometheus-docs,ddfreyne/prometheus-docs