Minor updates to the "Touch Knobs" example

Turn on anti-aliasing, and update screenshot.

The example is still not great and under-documented, but at least it
doesn't look horrible anymore.

Pick-to: 6.6
Task-number: QTBUG-120291
Change-Id: Ie709cb51d64c535d7cd0aa1451b3c5bb901324cb
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit 2b9d021ba5487f7f5f67b958a17160c4cf468561)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/widgets/touch/knobs/main.cpp

@@ -12,6 +12,7 @@ int main(int argc, char **argv)
 
     QGraphicsScene scene;
     QGraphicsView view(&scene);
+    view.setRenderHints(QPainter::Antialiasing);
 
     Knob *knob1 = new Knob;
     knob1->setPos(-110, 0);