use correct file extension when renaming

src/main.rs

@@ -33,8 +33,8 @@ fn rename_files(directory: &str, base: &str) -> Result<i32> {
                             let album = parse_album(&title);
                             count += 1;
                             let new_name = match album.len() {
-                                0 => format!("{artist}/{title}.mp4a"),
-                                _ => format!("{artist}/{album}/{title}.mp4b"),
+                                0 => format!("{artist}/{title}.m4b"),
+                                _ => format!("{artist}/{album}/{title}.m4b"),
                             };
                             println!("Renaming {} to {}", path.display(), new_name);
                             let target_path = Path::new(&base).join(new_name);