Fix preserveDrawingBuffer documentation

README.md

 - `autoplay` whether playback should start automatically after loading
 - `loop` whether playback is looped
 - `seekable` whether a seek-index is build during load time; neccessary for `seekToFrame` and `seekToTime` methods
-- `progressiveThrottled` whether to throttle downloading chunks until they're needed for playback. Requires `progressive`; default `false`.
+- `preserveDrawingBuffer` whether the WebGL context is created with preserveDrawingBuffer - necessary for "screenshots" via `canvas.toDataURL()`; default `false`.
 - `onload` a function that's called once, after the .mpg file has been completely loaded
 - `ondecodeframe` a function that's called after every frame that's decoded and rendered to the canvas
 - `onfinished` a function that's called when playback ends