Bug 20527: Corrected paid to amountwrittenoff

Bug 20527: Corrected paid to amountwrittenoff

Test Plan
1. Go to My account and go into accounting
2.Create manual invoice (If you already have invoices skip this step)
3.On the Make a payment tab click on Write off under actions of an
invoice
4. Click on Writeoff amount label and make sure that the amount written
is highlighted in green.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>