MP4 muxer.go failed with hevc codec

[tubzby]

Thanks for this project!

There's a minor error in format/mp4/muxer.go line 124:

		codec := self.CodecData.(h264parser.CodecData)

Should be:

		codec := self.CodecData.(h265parser.CodecData)

[Andrey Semochkin]

thx you