docs nbd: userspace NBD now favors github over sourceforge

While the sourceforge site for userspace NBD still exists, the code
repository moved to github several years ago.  Then with a recent
patch[1], the github landing page contains just as much information as
the sourceforge page, so we might as well point to a single location
that also provides the code.

[1] https://lists.debian.org/nbd/2023/03/msg00051.html

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20230410180611.1051618-5-eblake@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>