documentation post attack snapshot timestamp / date is incorrect

[ci-work]

https://github.com/terra-money/docs/blob/main/docs/migration/exchange-migration.md states
Post-attack snapshot to be taken at Terra Classic block 7790000: 2022.05.27 08:59:51 UTC

Proposal 1623 states: “Post-attack” snapshot to be taken at Terra Classic block 7790000 (2022.05.27 03:59:51+08:00)

Which converts to: 2022.05.26 19:59:51 UTC

@evanorti

[evanorti]

Thank you for your contribution!

I agree with your assessment. Because block height time predictions are not accurate, the block height itself will be emphasized in this guide.