Doc: Fix menu item name

This was changed back in the UI, because Stacked Container
is reserved for Stack Layout actions.

Change-Id: Id9fdf3ee6e36bdaa9afde492cfd60074d56da041
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

doc/src/qtquick/qtquick-designer.qdoc

@@ -154,7 +154,7 @@
     up.
 
     To change the stacking order of an item, right-click it in the navigator or
-    the \uicontrol {Form Editor} and select \uicontrol {Stacked Container}. You
+    the \uicontrol {Form Editor} and select \uicontrol {Stack (z)}. You
     can raise or lower the stack value of an item
     or move the item to the front or back of all its siblings. To remove the
     \c z property, select \uicontrol {Reset z Property}.