Update the getting-started to the new graphics library.

This fixes the code and makes it show the rotating box again.

But one question and one problem remain:

1. Is there still a `clear` function that clears the screen in the new graphics library? I couldn't find the equivalent in the new design.

2. I couldn't recreate the document (running `cargo run` from the root of the project) because of errors in the dependencies. I, however, have no idea how to fix them. Could someone take a look at it?