fix 'WalletUpdateSpent found spent coin' logging

When logging the spend of an output, show the output's value, not the sum off all my outputs in the partially spent transaction.

I have a 1000 CLAM output that I split into 200 5 CLAM outputs and left to stake.

When each one of them staked, I used to see:

    WalletUpdateSpent found spent coin 1000.00 CLAM [...]

which made it look like the whole 1000 CLAMs had been spent.

After this fix I see:

    WalletUpdateSpent found spent coin 5.00 CLAM [...]

which is more like the truth, and saves having to sum up all the outputs of the spent output's transaction too.