fix use of undefined function in file reader demo
fix use of undefined function in file reader demo

gpac/mp4box.js,gpac/mp4box.js,sjames1958gm/mp4box.js,hanzeil/mp4box.js,hanzeil/mp4box.js,b123400/mp4box.js,sjames1958gm/mp4box.js,b123400/mp4box.js