Update payments decimals/rounding policy in checklist

[0x3phemeralsoul]

https://github.com/sky-ecosystem/pe-checklists/blob/161895d52e144fec1e4d8cca52f1379ce63950cb/spell/spell-reviewer-mainnet-checklist.md#:~:text=Transfer%20amount%20is%20specified%20with%20(at%20least)%202%20decimals%20using%20ether%20keyword

Specifies decimaks for MKR, but we are now using SKY and also USDS.
Relevant discussion: https://discord.com/channels/893112320329396265/1285613415485931570/1287760500855406622