kivy/app.py should use an em dash instead of commas

[VicaWorth]

The :class:App class is the base for creating Kivy applications. Think of it as your main entry point into the Kivy run loop. In most cases, you subclass this class and make your own app. You create an instance of your specific app class and then**—when you are ready to start the application's life cycle—**you call your instance's :meth:App.run method.

Thank you!