Fix code example (#75)

Author: TamDik

README.md

@@ -42,7 +42,7 @@ fn main() -> Result<()> {
     println!("duration: {:?}", mp4.duration());
 
     // Track info.
-    for track in mp4.tracks().iter() {
+    for track in mp4.tracks().values() {
         println!(
             "track: #{}({}) {} : {}",
             track.track_id(),