fix(readme): change ng-file-model to ng-model

After trying to get a file-upload working I found out that `ng-file-model` is not used in combination with a `ng-file-select` and `ng-file-drop`. Instead, `ng-model` is used ([ng-file-select](https://github.com/danialfarid/angular-file-upload/blob/master/dist/angular-file-upload.js#L168) and [ng-file-drop](https://github.com/danialfarid/angular-file-upload/blob/master/dist/angular-file-upload.js#L252)). 

This PR fixes the examples in `README.md`.