error on example

in this example **cb** and **done** is undefined,
``
screenshot.render('Hello', function (err, data) {
// If there is an error close the web browser first before calling the
// errback
if (err) return screenshot.error(err, **cb**);

// data will contain a screenshot of the HTML as a node.js Buffer
console.log(data);
//

// Close the web browser (phantomjs or chrome)
screenshot.close(**done**);
});
``