Documentation/ABI/rnbd-clt: fix typo in sysfs-class-rnbd-client

/sys/block/rnbd<N> is created, not /sys/block/rnbd_client/rnbd<N>

Signed-off-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>