Added to_app call to the POD sample code.

Otherwise Plack complains when running the examples: “WARNING: Automatically
converting Plack::App::GitHub::WebHook instance to a PSGI code reference. If
you see this warning for each request, you probably need to explicitly call
to_app() i.e. Plack::App::GitHub::WebHook->new(...)->to_app in your PSGI file.”