readme: fix link to configuration section in the TOC
readme: fix link to configuration section in the TOC

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