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: 56f5160 ("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>

Documentation/block/ublk.rst

@@ -154,24 +154,30 @@ managing and controlling ublk devices with help of several control commands:
   How to deal with userspace/kernel compatibility:
 
   1) if kernel is capable of handling ``UBLK_F_UNPRIVILEGED_DEV``
+
     If ublk server supports ``UBLK_F_UNPRIVILEGED_DEV``:
+
     ublk server should send ``UBLK_CMD_GET_DEV_INFO2``, given anytime
     unprivileged application needs to query devices the current user owns,
     when the application has no idea if ``UBLK_F_UNPRIVILEGED_DEV`` is set
     given the capability info is stateless, and application should always
     retrieve it via ``UBLK_CMD_GET_DEV_INFO2``
 
     If ublk server doesn't support ``UBLK_F_UNPRIVILEGED_DEV``:
+
     ``UBLK_CMD_GET_DEV_INFO`` is always sent to kernel, and the feature of
     UBLK_F_UNPRIVILEGED_DEV isn't available for user
 
   2) if kernel isn't capable of handling ``UBLK_F_UNPRIVILEGED_DEV``
+
     If ublk server supports ``UBLK_F_UNPRIVILEGED_DEV``:
+
     ``UBLK_CMD_GET_DEV_INFO2`` is tried first, and will be failed, then
     ``UBLK_CMD_GET_DEV_INFO`` needs to be retried given
     ``UBLK_F_UNPRIVILEGED_DEV`` can't be set
 
     If ublk server doesn't support ``UBLK_F_UNPRIVILEGED_DEV``:
+
     ``UBLK_CMD_GET_DEV_INFO`` is always sent to kernel, and the feature of
     ``UBLK_F_UNPRIVILEGED_DEV`` isn't available for user