Fix chewie player example in chewie package on pub.dev

Currently the example on chewie links to [example/lib/main.dart](https://github.com/fluttercommunity/chewie/blob/master/example/lib/main.dart) whereas it should link to [example/lib/app/app.dart](https://github.com/fluttercommunity/chewie/blob/master/example/lib/app/app.dart) because of this it shows this in example section of pub.dev

If possible please simplify the example app as well. If needed I can create a PR with a simple example as well.

Compare the pub.dev example link with example/lib/main.dart and example/lib/app/app.dart, starting by checking how the example is rendered on pub.dev. Update the reference so it points to app/app.dart, then verify the rendered example shows the intended source; simplifying the app is optional and needs its scope clarified.

Written by the indexing model from the issue text.