Remove _addPromise from Dispatcher code example 

`_addPromise` is not used in the provided Dispatcher example, nor does it exist in Dispatcher.js found here: https://github.com/facebook/react/blob/master/examples/todomvc-flux/js/dispatcher/Dispatcher.js
Merge pull request #1892 from thauburger/patch-1

Remove _addPromise from Dispatcher code example