Use string instead of ol.OverlayPositioning enum in examples

examples/icon.js

@@ -1,6 +1,5 @@
 goog.require('ol.Map');
 goog.require('ol.Overlay');
-goog.require('ol.OverlayPositioning');
 goog.require('ol.View2D');
 goog.require('ol.layer.Tile');
 goog.require('ol.layer.Vector');
@@ -62,7 +61,7 @@ var element = document.getElementById('popup');
 
 var popup = new ol.Overlay({
   element: element,
-  positioning: ol.OverlayPositioning.BOTTOM_CENTER,
+  positioning: 'bottom-center',
   stopEvent: false
 });
 map.addOverlay(popup);