Update comment on deposit logic

[adlerjohn]

Outdated comment line:

https://github.com/FuelLabs/fuel/blob/afee682ddda10a5bb0f6cf4d349f426333464ca0/src/Deposit.yulp#L86

Need to update this line to describe the newer logic, i.e. only one deposit allowed.

Thanks to @homakov for noticing.