Correctness nitpick: H264 + AAC not muxed by WEBM

[klaxa]

While not relevant to explain concepts, I think correctness should still be thought about.
In the Transmuxing section the image depicts h264 + aac in a WEBM file. In reality this does not work as WEBM only supports VP8/VP9 and Vorbis/Opus. A container that can also mux H264 and AAC that is not MP4 would be MKV, which is also widely adopted.