LU-930 utils: minor fixes to lfs_migrate.1 man page

LU-930 utils: minor fixes to lfs_migrate.1 man page

Fix the formatting of the lfs_migrate.1 man page SYNOPSIS section,
since ".Blfs_migrate" is not the same as ".B lfs_migrate", and hence
the synopsis was missing the actual name of the command it described.

Also fix some minor grammar issues.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie0d8e3cd6fbab0663562b6a99f124ead953ebbe5
Reviewed-on: http://review.whamcloud.com/4440
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Richard Henwood <richard.henwood@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>