Use CargoCallbacks::new() in documentation

[george-hopkins]

Use CargoCallbacks::new() to fix:

warning: use of deprecated constant `bindgen::CargoCallbacks`: Use `CargoCallbacks::new()` instead. Please, check the documentation for further information.
  --> build.rs:11:44
   |
11 |         .parse_callbacks(Box::new(bindgen::CargoCallbacks))