When viewing the online version of the book at https://doc.rust-lang.org/book/, chapter 19-6 (https://doc.rust-lang.org/book/ch19-06-macros.html) ends with the text
To start defining the procedural macro, place the code in Listing 19-39 into your src/lib.rs file for the hello_macro_derive crate. Note that this code won’t compile until we add a definition for the impl_hello_macro function.
and what I assume to be the title legend for the code listing (which isn't there).
In the repository, the rest of the chapter is showing, so I believe this to be an issue with how the book generation happened.