Fix invalid ItemView example in docs

Looks like this was changed acknowledge the `underscore@1.7` template changes, however it nolonger passes the `variable: 'args'`

(90% sure) this should work in any version of underscore >= 1 
@thejameskyle, for older version of underscore template was `_.template(templateString, data, settings)`. In `1.7` we decided to respect that case and handle the 3 args settings. `null` is because internally in template they used to use `if (data) return render(data);` (< 1.7.0).