Fix example

The `next` reference is missing in the sample:

```
var encoder = mp4.encode()
var decoder = mp4.decode()

decoder.on('box', function (headers, next) {
decoder.decode(function (box) {
encoder.box(box, next)
})
})

fs.createReadStream('my-movie.mp4').pipe(decoder)
encoder.pipe(fs.createWriteStream('my-movie-copy.mp4'))
```

Start by locating the documentation or example containing the `mp4.encode()` and `mp4.decode()` snippet from the issue. Check how the decoder callback receives `next`, update the sample so it is included, and verify that the documented stream-copy example is complete.

Written by the indexing model from the issue text.