message: Fix typo in book
  While reading I found a little typo in the docs and corrected it.
diff: @@ -27,7 +27,7 @@ bench = false
  ### Binaries
  -Binary targets are executables programs that can be run after being compiled.
  +Binary targets are executable programs that can be run after being compiled.
  The default binary filename is `src/main.rs`, which defaults to the name of
  the package. Additional binaries are stored in the [`src/bin/`
  directory][package layout]. The settings for each binary can be [customized]
  