Fix broken link to non-existent class

Change QQuickCanvas to QQuickWindow as the class was renamed.

Task-number: QTBUG-28795
Change-Id: Id3dfa2242ca60dc97c8195d86d81531148ac85d3
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

doc/src/whatsnew/whatsnew50.qdoc

@@ -174,7 +174,7 @@
     following:
     \list
      \li QQuickView
-     \li QQuickCanvas
+     \li QQuickWindow
      \li QQuickItem
      \li QQuickPaintedItem
     \endlist