KAbstractFileItemActionPlugin: Add missing quotes in code example

Summary: The quotes around the json file name were added. Without these the moc process would throw a parse error.

Test Plan: /

Reviewers: elvisangelaccio, Frameworks, mlaurent, apol

Reviewed By: elvisangelaccio, mlaurent, apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27591