Docs: Fix example code error

Author: myyrakle

Diff for: book/src/tutorial-3.md

@@ -30,7 +30,7 @@ fn main() {
         .header("wrapper.h")
         // Tell cargo to invalidate the built crate whenever any of the
         // included header files changed.
-        .parse_callbacks(Box::new(bindgen::CargoCallbacks::new()))
+        .parse_callbacks(Box::new(bindgen::CargoCallbacks))
         // Finish the builder and generate the bindings.
         .generate()
         // Unwrap the Result and panic on failure.