Improve doc for the non-draggable draggableProgressBar please

[ThomasGysemans]

The documentation of the ChewieController 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.