fix: fix assertion

[YichiZhang0613]

The inconsistency among comment, code and assertion cause confusion here, which should be fixed.
The assumption("ASSUMPTION: our validity period is AT LEAST 2 days") indicates that TICKETBOOK_VALIDITY_DAYS >= 2 days, which indicates TICKETBOOK_VALIDITY_DAYS - 1 >= 1 day. So target_secs should be >= 86400.
The comment will cause the confusion("making sure it's no greater than 1 day"). It indicate that it should be <= 86400, which is wrong semantically and should be changed it into "no lower than 1 day".
The assertion message requires target_secs >= 86400("no lower than"), while the predicate is target_secs > 86400, so I changed it into assert!(target_secs >= 86400,"the maximum time between redemption can't be lower than 1 day!");.

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