zpoolprops.7: Fix max length of name of user property

The documentation mentioned that the property name can be 256 characters
long. This was incorrect. The last byte is reserved for NUL, so the
name provided by the operator can be only 255 characters long.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Mateusz Piotrowski <0mp@FreeBSD.org>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #16248