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

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