docs(README): fix the link to the changelog in the Cargo book (#15597)

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### What does this PR try to resolve?

When Cargo's changelog was moved into the book in #15123, it seems the
extension was left as `.md`, whereas the link should point to the
rendered version, which ends with `.html`.

### How should we test and review this PR?

Following the new link in the README's rendered view and observing that
it does indeed point to the changelog should be enough.

Author: weihanglo

README.md

@@ -92,7 +92,7 @@ High level release notes are available as part of [Rust's release notes][rel].
 Detailed release notes are available in the [changelog].
 
 [rel]: https://github.com/rust-lang/rust/blob/master/RELEASES.md
-[changelog]: https://doc.rust-lang.org/nightly/cargo/CHANGELOG.md
+[changelog]: https://doc.rust-lang.org/nightly/cargo/CHANGELOG.html
 
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