docs: DirView example uses QFileSystemModel not QDirModel

Change-Id: Ie172b1458401c4627314ff0db6992647388bf382
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

examples/widgets/doc/src/dirview.qdoc

@@ -32,7 +32,7 @@
     \brief This example demonstrates the usage of a tree view.
 
     \brief The Dir View example shows a tree view onto the local filing system. It uses the
-    QDirModel class to provide file and directory information.
+    QFileSystemModel class to provide file and directory information.
 
     \image dirview-example.png
 */