accounts: fix compilation issue for i386 systems

Apparently the %d formatting directive implicitly uses `int` as its data
type, which on i386 systems is int32. So the value math.MaxInt64
overflows that value, which causes the compilation to fail.

accounts/store_kvdb.go

@@ -244,7 +244,7 @@ func (s *BoltStore) IncreaseAccountBalance(_ context.Context, id AccountID,
 
 	update := func(account *OffChainBalanceAccount) error {
 		if amount > math.MaxInt64 {
-			return fmt.Errorf("amount %d exceeds the maximum of %d",
+			return fmt.Errorf("amount %d exceeds the maximum of %v",
 				amount, math.MaxInt64)
 		}