Switch to short form for serde.rs link in rustdoc

Fixes a rustdoc warning:

    warning: unneeded long form for URL
      --> serde/src/lib.rs:12:27
       |
    12 | //! See the Serde website [https://serde.rs/] for additional documentation and
       |                           ^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://serde.rs/>`
       |
       = note: `#[warn(non_autolinks)]` on by default

serde/src/lib.rs

@@ -9,11 +9,9 @@
 //! these two groups interact with each other, allowing any supported data
 //! structure to be serialized and deserialized using any supported data format.
 //!
-//! See the Serde website [https://serde.rs/] for additional documentation and
+//! See the Serde website <https://serde.rs/> for additional documentation and
 //! usage examples.
 //!
-//! [https://serde.rs/]: https://serde.rs/
-//!
 //! ## Design
 //!
 //! Where many other languages rely on runtime reflection for serializing data,