xfs_io: clarify the fsmap documentation

Explicitly declare that the 'start' and 'end' arguments to fsmap require
one of -d, -l, or -r to select the data, log, or realtime device, and fix
misspelled command name while we're at it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>