Doc: Use nullptr instead of 0 in example

Pick-to: 5.15
Fixes: QTBUG-84101
Change-Id: Ia0b5a87bf9776ee14eff0483b955dd99389cb6ae
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

examples/widgets/painting/basicdrawing/renderarea.h

@@ -65,7 +65,7 @@ public:
     enum Shape { Line, Points, Polyline, Polygon, Rect, RoundedRect, Ellipse, Arc,
                  Chord, Pie, Path, Text, Pixmap };
 
-    RenderArea(QWidget *parent = 0);
+    explicit RenderArea(QWidget *parent = nullptr);
 
     QSize minimumSizeHint() const override;
     QSize sizeHint() const override;