doc: Update COPYRIGHT link

[reddevilmidzy]

The previous link is no longer valid. After checking the Wayback Machine, I found that its contents are similar to https://github.com/rust-lang/rust/blob/master/COPYRIGHT

[Stebalien]

From what I can tell, that entire (second) sentence can be cut. It's left over from when this file was a part of the rust project itself. At the time, they needed a separate file to explain the different copyright status/licenses of all the files in the repo.

In this case:

1. This one file is copyright the rust project developers (already stated at the top).
2. This file (and the project) are all licensed under the same MIT OR Apache-2.0 license.

[reddevilmidzy]

Thank you for the review:)

[Stebalien]

Thanks!