Bug 31526: Follow-up for bug 29697

Bug 31526: Follow-up for bug 29697

There was a typo in bug 29697 which lead to a code reference error.

Test plan
1) Export a record with ISBN from your catalog (Save as MARC)
2) Stage it using the ISBN matching rule (make sure it matched)
3) Manage staged import and click on the Diff link

Prior to this patch an error would be thrown, after the patch you should
now get the diff view as expected again.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>