Bug 24462: Adapt mapping to voted RFC (Invoices)

Bug 24462: Adapt mapping to voted RFC (Invoices)

This trivial patch changes the attribute names for API reprensentation
of Koha::Acquisition::Invoice objects.

To test:
1. Open https://wiki.koha-community.org/wiki/Acquisitions_invoices_endpoint_RFC
2. Check the mappings are correct
=> SUCCESS: The patch makes the right changes to the mappings
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>