QQmlEngine - fix example in documentation

example_qjsvalue_singletontype_provider example code snippet
returns QJSValue instead of QJSValue*.

Change-Id: If4d5592fcee25ada9870602c8f92197980ef9b77
Reviewed-by: Nico Vertriest <nico.vertriest@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>