btrfs-progs: random fixes for usage string of btrfs scrub

btrfs-progs: random fixes for usage string of btrfs scrub

- Add missing description about "-R" option in the command
  usage of "btrfs scrub resume".

- Add missing comma to avoid the following misformatted command
  usage of "btrfs scrub start". See the line of "-R" option.

===
usage: btrfs scrub start [-BdqrRf] [-c ioprio_class -n ioprio_classdata] <path>|<device>

    Start a new scrub

    -B     do not background
    -d     stats per device (-B only)
    -q     be quiet
    -r     read only mode
    -R     raw print mode, print full data instead of summary-c     set ioprio class (see ionice(1) manpage)
    -n     set ioprio classdata (see ionice(1) manpage)
    -f     force to skip checking whether scrub has started/resumed in userspace
           this is useful when scrub stats record file is damaged
===

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>