message: Fix minor typo in the book.
diff: @@ -626,7 +626,7 @@ You can run individual executable examples with the command `cargo run --example
  Specify `crate-type` to make an example be compiled as a library (additional
  information about crate types is available in
  -[the Cargo reference](https://doc.rust-lang.org/reference/linkage.html)):
  +[The Rust Reference](https://doc.rust-lang.org/reference/linkage.html)):
  ```toml
  [[example]]
  