fix bug in boxAdaptor code sample

still, `width` and `height` seem to be undefined, maybe they should be `data.width` and `data.height`?
I don't know how to run that part of the adaptor code in the (JSFiddle)[http://jsfiddle.net/rich_harris/ATAgH/] without deleting line 169, so I'm not sure how it's supposed to work.