docs/archimport: quote sourcecontrol.net reference

git-archimport has an option to register archives at mirrors.sourcecontrol.net. The sourcecontrol.net domain still exists, but that hostname no longer exists. That means this feature is presumably broken. I'll leave the examination and modification of that to people who might actually use archimport. But in the meantime, let's wrap the reference in the documentation in backticks, which will avoid turning it into a broken link (and thus polluting linkchecker results). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>