Update #api_authenticate example in README.md

[nativestranger]

This example was broken because before_filters & before_actions need to render or redirect in order to prevent actions from being called.

The example also uses a before_action now since it is preferred over before_filter.

Let me know if you'd like to merge this but want me to make some changes first!

[awendt]

Thanks for catching the "render or redirect, not return false" part! :+1:

[kjg]

As pointed out by @ShadowRadiance in #79, let's make this a head(:unauthorized), and then I think this is good to go.

Thanks everyone!

[nativestranger]

Okay @kjg good to go! :+1: