[web-midi-api] Add (non-normative) reference to Web Audio API (#83)

The Web Audio API is referred to a few times in the spec, but missing in the references section. It would be particularly useful to have in Section 13.7 A Simple Monophonic Sine Wave MIDI Synthesizer.

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Reply to this email directly or view it on GitHub:
https://github.com/WebAudio/web-midi-api/issues/83