Typo: major version is set twice, while minor version is not set

[Steffo99]

Here, specifically:

learn-opengl/book-src/basics/000-creating-a-window.md

Lines 27 to 28 in 4e2a113

	sdl.set_gl_context_major_version(3).unwrap();
	sdl.set_gl_context_major_version(3).unwrap();

[Lokathor]

This is definitely a bug. I'd merge a PR to fix it, or will try to find the time myself if you can't.