Fix incorrect links (rust-lang#1549)

Author: mu001999

src/serialization.md

@@ -162,5 +162,5 @@ for `Encodable<CacheEncoder>`.
 [`rustc_metadata::rmeta::decoder::DecodeContext`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/decoder/struct.DecodeContext.html
 [`rustc_metadata::rmeta::encoder::EncodeContext`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/rmeta/encoder/struct.EncodeContext.html
 [`rustc_serialize`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_serialize/index.html
-[`TyDecoder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.TyEncoder.html
-[`TyEncoder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.TyDecoder.html
+[`TyDecoder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.TyDecoder.html
+[`TyEncoder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/codec/trait.TyEncoder.html