Fix build by making underline long enough

Author: tannewt

README.rst

@@ -40,7 +40,7 @@ This plays Frosty the Snowman:
  adafruit_rtttl.play(board.A0, "Snowman:d=8,o=5,b=200:2g,4e.,f,4g,2c6,b,c6,4d6,4c6,4b,a,2g.,b,c6,4d6,4c6,4b,a,a,g,4c6,4e.,g,a,4g,4f,4e,4d,2c.,4c,4a,4a,4c6,4c6,4b,4a,4g,4e,4f,4a,4g,4f,2e.,4e,4d,4d,4g,4g,4b,4b,4d6,d6,b,4d6,4c6,4b,4a,4g,4p,2g")
 
 CPX Usage Example
-=============
+=================
 
 This plays Frosty the Snowman on a Circuit Playground Express (we must enable onboard speaker):