fix(tutorial): use p5.Oscillator constructor

Fixes a ReferenceError in the simple-melody-app tutorial where Oscillator was used without the p5 namespace, causing the example to fail at runtime.

Author: aashu2006

src/content/tutorials/en/simple-melody-app.mdx

@@ -295,7 +295,7 @@ let myFreq = 262;
 function setup() {
   createCanvas(400, 400);
   // Create the Oscillator object.
-  osc = new Oscillator(myFreq);
+  osc = new p5.Oscillator(myFreq);
   // console.log(osc.f);
 }