piano: add square wave for PicoSystem beepy audio

examples/piano/piano.cpp

diff --git a/examples/piano/piano.cpp b/examples/piano/piano.cpp
index 8df5994..f39396a 100644
--- a/examples/piano/piano.cpp
+++ b/examples/piano/piano.cpp
@@ -36,7 +36,7 @@
   // configure voices

 

   // melody track

-  channels[0].waveforms   = Waveform::TRIANGLE;

+  channels[0].waveforms   = Waveform::SQUARE;

   channels[0].attack_ms   = 10;

   channels[0].decay_ms    = 350;

   channels[0].sustain     = 0;