README.md: Fix the FM example to clear amp[VEL].

[dpwe]

The example that plays a couple of FM tone by directly configuring the individual sine oscillators had been broken since we made the change that unspecified ControlCoefficients are left unchanged (rather than being cleared to zero). Because FM component oscs do not see the velocity of their parent note, it's important to include amp={'vel': 0} (to clear the default weight of 1), otherwise the operator oscs will always have zero output, because they see a velocity of zero.

Doing this, I realize that ratio is really just a special case of ControlCoefficients. Instead of ratio=0.2 (and the associated explanation), that's really just freq={'const': 52.326, 'note': 1}, i.e. it follows the note frequency, but relative to 0.2 x the default frequency (of 261.63 Hz). We could change that. Would it make things easier to understand?

[dpwe]

I realize using the "dict" form of specifying ControlCoefficients will weird-out people unfamiliar with Python. At the same time, it's actually a lot more comprehensible than '1,0,0,1' once you learn the syntax.