Improve doc for the non-draggable draggableProgressBar please

The documentation of the [ChewieController](https://pub.dev/documentation/chewie/latest/chewie/ChewieController-class.html) class mentions the presence of a property that suggests to the developer that a draggable progress bar feature exists. I was surprised to discover that this `draggableProgressBar` property shows a non-draggable progress bar... I'm suggesting that this property's name should be changed into `nonDraggableProgressBar` for clarity's sake.