[TodoView] Use parents collection when creating a subtodo

Summary:
When creating a subtodo of an existing task, it should belong to the same collection.

Apparenly, there was a typo: parent collection was determined, but not passed to `createIncidence` call.
Just fix it.

Test Plan:
1) Open KOrganizer todo list, select an existing todo
2) Enter new todo description in the text widget below the list, press Ctrl+Enter
3) (without patch) It asks for a collection despite attempting to create a sub-todo
4) (with patch) A sub-todo gets created in the same collection

Reviewers: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28434