LU-11099 doc: include "-N" option to lfs_setstripe.1

LU-11099 doc: include "-N" option to lfs_setstripe.1

This patch includes mirror option "-N[mirror_count]" to
lfs_setstripe.1 man page so that the user can follow the manual
to create a mirrored file or set s default mirror layout on a
directory correctly.
The command format is like:
$lfs setstripe -N[mirror_count] [STRIPE_OPTIONS] <dir|filename>

Test-Parameters: trivial

Change-Id: If0fabd79d218e5582f9c64336f60466f35dbd968
Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/32734
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>