Update env.js

fix undeclared variable in example

Author: Spongman

src/env.js

@@ -158,7 +158,7 @@ define(function (require) {
    *  var t2 = 0.3; // decay time in seconds
    *  var l2 = 0.1; // decay level  0.0 to 1.0
    *  var t3 = 0.2; // sustain time in seconds
-   *  var l3 = dL; // sustain level  0.0 to 1.0
+   *  var l3 = 0.5; // sustain level  0.0 to 1.0
    *  // release level defaults to zero
    *
    *  var env;