[Interesting Taps and Forks] Update unmaintained ffmpeg tap to Current

[Amar1729]

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Minor change: the current tap for ffmpeg on Interesting Taps and Forks is outdated.
Homepage for varenc/ffmpeg notes that this tap is not currently maintained, and to use homebrew-ffmpeg/ffmpeg instead.

[MikeMcQuaid]

Thanks @Amar1729!

[MikeMcQuaid]

Thanks so much for your contribution! Without people like you submitting PRs we couldn't run this project. You rock, @Amar1729!