Remove www subdomain from Prometheus link.
Remove www subdomain from Prometheus link.

The prometheus link in the README is broken - there is no page at www.prometheus.io (it's at promethius.io - I guess they didn't set up a redirect). @beorn7
KevinPike/client_golang,asbjornenge/client_golang,stapelberg/client_golang,m3db/prometheus_client_golang,jimmidyson/prometheus-client_golang,mattkanwisher/client_golang,prometheus/client_golang