message: Fix cargo doc warning
diff: //! If the packet has no payload, you must still specify this attribute, but you can provide a
  //! `#[length_fn]` attribute returning zero.
  //!
  -//! * \#[construct_with(<primitive type>, ...)]
  +//! * \#[construct_with(\<primitive type>, ...)]
  //!
  //! Unfortunately, compiler plugins do not currently have access to type information during the
  //! decoration stage (where all of the above is generated), so this attribute is required. This
  