Fix "listen" directive syntax prometheus.conf (#180)

There's a syntax issue in the "listen" directive. Should be "listen 9113;", not "listen 9113:". Using the current file, I got an error upon reloading the NGINX service ("nginx: [emerg] invalid port in "9113:" of the "listen" directive in /etc/nginx/conf.d/prometheus.conf:11")

Author: itaytalmi