Fix broken links.

[meowjesty]

• wgpu-rs was linking to the archived version;
• rust-tutorials/learn-gfx-hal doesn't exist anymore;

As there is only 1 tutorial link for gfx, I've changed plurals to singular as well.

This is a minor change, does it need an issue?

[cwfitzgerald]

Thanks!

bors r+

[bors[bot]]

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