Use style option in vector layer examples

Providing a styleFunction option still works because the layer property is still named styleFunction.

examples/drag-and-drop.js

@@ -118,7 +118,7 @@ dragAndDropInteraction.on('addfeatures', function(event) {
   });
   map.getLayers().push(new ol.layer.Vector({
     source: vectorSource,
-    styleFunction: styleFunction
+    style: styleFunction
   }));
   var view2D = map.getView().getView2D();
   view2D.fitExtent(vectorSource.getExtent(), map.getSize());