Format README of vp9 decoder module
Format README of vp9 decoder module

PiperOrigin-RevId: 378390492

amzn/exoplayer-amazon-port,google/ExoPlayer,amzn/exoplayer-amazon-port,androidx/media,google/ExoPlayer,amzn/exoplayer-amazon-port,androidx/media,ened/ExoPlayer,ened/ExoPlayer,ened/ExoPlayer,androidx/media,google/ExoPlayer