Documents links 404

[valarauca]

In both the README.md and on Crates.io

[steveklabnik]

Hm, https://doc.rust-lang.org/libc resolves for me.

[valarauca]

Click any of the links below that.

[alexcrichton]

This was likely due to rust-lang/rust#44487 and should get fixed in the near future