Fix syntax error.

[shimabox]

I tried to get the elapsed time, but it was NaN.

<script>
window.onload = function(){
  var ctx = document.getElementsByTagName('canvas')[0].getContext('2d')
  var draw = function(video, dt) {
    console.log(dt); // => NaN
    ctx.drawImage(video, 0, 0)
  }
  var myCamvas = new camvas(ctx, draw)
}
</script>

I fixed it for that.

[shimabox]

@cbrandolino Thank you for approval. I would be happy if you merge it when you are free.

[shimabox]

@cbrandolino Thank you for a nice library!