QGraphicsView: clarify documentation for viewportTransform

d->matrix holds the scene->viewport transform, not vice versa.

Change-Id: I997cbdfbf519f39f242cd0dbbd00621e59ec9307
Task-number: QTBUG-48705
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>

src/widgets/graphicsview/qgraphicsview.cpp

@@ -3828,7 +3828,7 @@ QTransform QGraphicsView::transform() const
 }
 
 /*!
-    Returns a matrix that maps viewport coordinates to scene coordinates.
+    Returns a matrix that maps scene coordinates to viewport coordinates.
 
     \sa mapToScene(), mapFromScene()
 */