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

@@ -32,6 +32,7 @@ AudioSynthWaveformSine	KEYWORD2
 AudioSynthWaveformSineModulated	KEYWORD2
 AudioSynthWaveformDc	KEYWORD2
 AudioSynthNoiseWhite	KEYWORD2
+AudioSynthNoisePink	KEYWORD2
 isPlaying	KEYWORD2
 positionMillis	KEYWORD2
 lengthMillis	KEYWORD2