kolab_api_service: fix typo in variable name

Summary: Fixes typo to make object_type detection works again. This fixes T6694

Test Plan:
Create a shared calendar folder via webadmin. Try to edit
the previously created shared folder. The form for shared addressbook
and its title is shown.

Reviewers: Web Administration Panel Developers, vanmeeuwen, machniak

Reviewed By: Web Administration Panel Developers, machniak

Maniphest Tasks: T6694

Differential Revision: https://git.kolab.org/D2860