Fix txg_log_time ZAP key typo

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Igor Ostapenko <igor.ostapenko@klarasystems.com>
Sponsored-by: Klara, Inc.
Closes #17748