readme: fix formatting of the configuration section
readme: fix formatting of the configuration section

korczis/docker-squid-rubygems-proxy,maqsoodumt/docker-squid,linearregression/docker-squid-s3,boliu83/docker-squid,Optibus/docker-squid,sameersbn/docker-squid,korczis/docker-squid-rubygems-proxy,opendoor-labs/docker-squid-s3,zultron/docker-provisioning