The current incorrect link at: https://www.gnu.org/software/guix/manual/en/guix.html#Running-GuixSD-in-a-VM

.

I looked for other instances of this mistake in the docs, but I could't
find any.

* doc/guix.texi: fix link to GuixSD VM image in FTP server.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>