fsck.f2fs: Update the usage about option of preen mode

Update the usage about option of preen mode.
It's difficult to know about preen mode 2, because there is no
description in usage.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
1 file changed