Add import to window::Window in main.rs

[luhem7]

This is a minor issue

I'm on /beginner/tutorial2-swapchain/#first-some-house-keeping. While following along with the tutorial, I realized that I needed to import window::Window along with window::WindowBuilder.

[luhem7]

Oh my bad, I see the code in the chapter source code. I'm going to close out this issue, but feel free to update the code in the chapter itself!