Fix a mis-merge in the zdb man page (#16304)

Sponsored-by: Klara, Inc.
Sponsored-By: Wasabi Technology, Inc.

Signed-off-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>