Adding keyword for AudioSynthNoisePink

Fixes the syntax highlighting in the Arduino IDE. This is my first time using Github's web editor, hopefully the whitespace comes out okay.

keywords.txt

 AudioSynthWaveformSineModulated	KEYWORD2
 AudioSynthWaveformDc	KEYWORD2
 AudioSynthNoiseWhite	KEYWORD2
+AudioSynthNoisePink	KEYWORD2
 isPlaying	KEYWORD2
 positionMillis	KEYWORD2
 lengthMillis	KEYWORD2