Fix broken link to rustc_type_ir module in serialization docs

Author: davidjsonn

src/doc/rustc-dev-guide/src/serialization.md

@@ -106,7 +106,7 @@ type wrapper, like [`ty::Predicate`] and manually implementing `Encodable` and
 [`Encodable`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_serialize/trait.Encodable.html
 [`Encoder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_serialize/trait.Encoder.html
 [`RefDecodable`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.RefDecodable.html
-[`rustc_middle`]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_type_ir/codec.rs.html#21
+[`rustc_middle`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/index.html
 [`ty::Predicate`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/predicate/struct.Predicate.html
 [`TyCtxt`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html
 [`TyDecodable`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_macros/derive.TyDecodable.html