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>

Author: peff

Documentation/git-archimport.txt

@@ -96,7 +96,7 @@ OPTIONS
 	pruned.
 
 -a::
-	Attempt to auto-register archives at http://mirrors.sourcecontrol.net
+	Attempt to auto-register archives at `http://mirrors.sourcecontrol.net`
 	This is particularly useful with the -D option.
 
 -t <tmpdir>::