Update freebsd.md

Open username-removed-709879 requested to merge ZeWaren/gitlab-ci-multi-runner:patch-1 into master
  • Put the rc script in /usr/local/etc/rc.d since it's not a system script but a local one.
  • Use fetch to download the runner executable, since fetch is provided by the FreeBSD base system, and wget is a Linux tool that needs to be installed from ports.