Bug 19073: (bug 15758 follow-up) Dereference correct value from Koha::Object.

Bug 19073: (bug 15758 follow-up) Dereference correct value from Koha::Object.

* tools/modborrowers.pl: Dereference 'branchcode' rather than
  'value'.

Test Plan:
- Carry out "Batch patron modification" with any number of patrons.
  + Change the library.
  + Hit Save.
- The library will not have been updated.
- Apply patch.
- Run same modification.
- The library will have been updated

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>