Minor verbiage update for private repositories

Hi there,

I was just using Octotree on a private repository and thought the text `Why need access token?` might read better as `Why is this required?`. Patch attached for your consideration.

Cheers,
  Lee :beers:

![github_github](https://cloud.githubusercontent.com/assets/121322/3033425/17260692-e064-11e3-8ec6-2967abfc6496.png)