Fix broken link to rustc_type_ir module in serialization docs

[davidjsonn]

PR fixes a broken link in the Rust compiler documentation. The original link pointed to rustc_type_ir/codec.rs, which no longer exists in the current compiler architecture. This functionality has been moved to the rustc_middle crate.

The change updates the broken link:

• From: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_type_ir/codec.rs.html#21
• To: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/index.html

[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

[fee1-dead]

r=me and errs once the suggestion is applied.

[davidjsonn]

What would you suggest doing?

[fee1-dead]

errs gave a suggestion in his review

[davidjsonn]

done, thank you!