[types] update comment about transaction expiration times

[bob-wilson]

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.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

N/A

[bob-wilson]

/land

[github-actions]

Cluster Test Result

Compatibility test results for land_0e945e2e ==> land_759dc06a (PR)
1. All instances running land_0e945e2e, generating some traffic on network
2. First validator land_0e945e2e ==> land_759dc06a, to validate storage
3. First batch validators (14) land_0e945e2e ==> land_759dc06a, to test consensus
4. Second batch validators (15) land_0e945e2e ==> land_759dc06a, to upgrade rest of the validators
5. All full nodes (30) land_0e945e2e ==> land_759dc06a, to finish the network upgrade
all up : 995 TPS, 4545 ms latency, 5500 ms p99 latency, no expired txns
Logs: http://kibana.ct-2-k8s-testnet.aws.hlw3truzy4ls.com/app/kibana#/discover?_g=(time:(from:'2020-08-25T22:26:03Z',to:'2020-08-25T22:51:50Z'))
Dashboard: http://grafana.ct-2-k8s-testnet.aws.hlw3truzy4ls.com/d/2XqUIhnWz/performance?from=1598394363000&to=1598395910000
Validator 1 logs: http://kibana.ct-2-k8s-testnet.aws.hlw3truzy4ls.com/app/kibana#/discover?_g=(time:(from:'2020-08-25T22:26:03Z',to:'2020-08-25T22:51:50Z'))&_a=(columns:!(log),query:(language:kuery,query:'kubernetes.pod_name:"val-1"'),sort:!(!('@timestamp',desc)))

Repro cmd:

./scripts/cti --tag land_0e945e2e --cluster-test-tag land_759dc06a -E RUST_LOG=debug -E BATCH_SIZE=15 -E UPDATE_TO_TAG=land_759dc06a --report report.json --suite land_blocking_compat