block: ublk: fix doc build warning

Fix the following warning:

Documentation/block/ublk.rst:157: WARNING: Enumerated list ends without a blank line; unexpected unindent.
Documentation/block/ublk.rst:171: WARNING: Enumerated list ends without a blank line; unexpected unindent.

Fixes: 56f5160bc1b8 ("ublk_drv: add mechanism for supporting unprivileged ublk device")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230118042318.127900-1-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>