doc: Fix variable naming inconsistency in MP4Box initialization

[gspinoza]

Description

updated README.md to fix a variable naming inconsistency mistake in mp4boxfile initialization. The mistake is in the code for the Segmentation example.

should be:

var mp4boxfile = MP4Box.createFile();
mp4boxfile.onReady = function(info) {
  ...
};

not:

var mp4box = MP4Box.createFile();
mp4boxfile.onReady = function(info) {
  ...
};

Changes

Renamed the variable mp4box to mp4boxfile for consistency throughout the codebase.

[cconcolato]

Thank you.