Fix '-T u|d' descriptions in zpool(8)

In

-T u|d Display a time stamp. Specify -u for a printed

		representation of the internal representation of time.
		See time(2).  Specify -d for standard date format.
		See date(1).

'Specify u' and 'Specify d' should be used instead. zpool list -T -u
does not work.

Bring the descriptions in zpool list and zpool status in sync with
zpool iostat.

Reviewed by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Anatoly Borodin <anatoly.borodin@gmail.com>
Closes #8438