Clear up confusion in readme.md about initializing mixpanel

[fotoflo]

The docs say:

angular.module('myApp', ['analytics.mixpanel']);

Minimally you'll need to configure your API key like so:

 angular.module('analytics.mixpanel')
        .config(['$mixpanelProvider', function($mixpanelProvider) {
            $mixpanelProvider.apiKey('<your project token>'); // your token is different than your API key
        }]);

However I think the second module should also be 'myApp'