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 <[email protected]>
Reviewed-by: Josef Bacik <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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