simpletreemodel example: rename parent() to parentItem() for clarity

I show this example during trainings, and parent() is too much like
QObject::parent(), QModelIndex::parent() and our model's parent()
[which calls this method], so it's less confusing if this is called
something clearer like parentItem().

Change-Id: I101342051349d94c4a3bc3d4bc332194d6779293
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>