Fixes mixingSound Example

Replaced calls to connectSource with setInput.

This example was still calling p5.Gain.connectSource instead of
p5.Gain.setInput introduced in commit
41d6d1d causing the sketch not to run.

Author: TheFernDawg

examples/mixingSounds/sketch.js

@@ -22,15 +22,15 @@ function setup() {
   sound1.disconnect(); // diconnect from p5 output
 
   gain1 = new p5.Gain(); // setup a gain node
-  gain1.connectSource(sound1); // connect the first sound to its input
+  gain1.setInput(sound1); // connect the first sound to its input
   gain1.connect(gain3); // connect its output to the 'master'
 
   sound2.rate(1);
   sound2.disconnect();
   sound2.loop();
 
   gain2 = new p5.Gain();
-  gain2.connectSource(sound2);
+  gain2.setInput(sound2);
   gain2.connect(gain3);
 
 }