doc/cdrom: Trvial documentation error, file not present

The sbpcd tester program is not included in the kernel source tree,
so remove the reference to it.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Reported-by: Nick Warne <nick@ukfsn.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>