Docs: fix bad example code

Summary:

• setupBindings() doesn't take an engine parameter
• setupBindings() is deprecated and suggests to call setupContext() instead, although you need to setup the engine as well and tell KDeclarative to use that engine.

Test Plan: - the previous example code wouldn't compile

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22765