mkfs.f2fs: change -c option description

Current description confuses users like they can add addtional devices
with one -c option using commas(,) at the same time.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
1 file changed