(state statute allowing timeframe for buyer of real estate to rescind purchase contract once seller provides buyer with affidavit concerning certain statutorily required attributes of the land upheld as constitutional and seller compliance with statute is mandatory unless waived by buyer; as applied to buyers in the case who had actual knowledge of a federal flood easement over the property at issue, two buyers allowed their statutory rescission rights to lapse and are bound by the purchase contracts, but third buyer properly invoked rescission right).