[types] update comment about transaction expiration times
[types] update comment about transaction expiration times

After #5674, the maximum value for transaction expiration is no
longer constrained by a factor of 1_000_000, so update the comment
to reflect that.

Closes: #5767

aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core