docs: fix dead link

[GarmashAlex]

I have fixed the link to the documentation for unsized_tuple_coercion. Now, instead of the broken link https://doc.rust-lang.org/beta/unstable-book/language-features/unsized-tuple-coercion.html, the current link https://doc.rust-lang.org/stable/unstable-book/language-features/unsized-tuple-coercion.html is used.

[rustbot]

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[rustbot]

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol

[Zalathar]

Thanks for your efforts, but switching over to the stable URL is not appropriate in this case, because the relevant feature has been removed and the updated link will soon become dead too.

Closing in favour of rust-lang/rustc-dev-guide#2382.