Fix broken link in error code E0706 docs

Corresponding subsection in async book is not `07.05` not `07.06`.

The information on the linked page is the same so it may be reasonable to remove the whole sentence.

compiler/rustc_error_codes/src/error_codes/E0706.md

@@ -56,4 +56,4 @@ You might be interested in visiting the [async book] for further information.
 [`async-trait` crate]: https://crates.io/crates/async-trait
 [async-is-hard]: https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/
 [Generic Associated Types]: https://github.com/rust-lang/rust/issues/44265
-[async book]: https://rust-lang.github.io/async-book/07_workarounds/06_async_in_traits.html
+[async book]: https://rust-lang.github.io/async-book/07_workarounds/05_async_in_traits.html