message: Add missing db param in book example
diff: @@ -114,7 +114,7 @@ Finally, you can also modify the value of an input field by using the setter met
  Since this is modifying the input, the setter takes an `&mut`-reference to the database:
  ```rust
  -file.set_contents(String::from("fn foo() { /* add a comment */ }"));
  +file.set_contents(&mut db, String::from("fn foo() { /* add a comment */ }"));
  ```
  ## Tracked functions
  