Fix computation of half wavelength in I2s-library-examples.md

Dividing the sampleRate by the frequency results in the number of samples per complete wave. For example: with a sampling rate of 24 and a 4Hz frequency we have 24/4 -> 6 samples, which represent a wave peak to peak; we still need to divide by 2 to have its half.

Author: Narcolessico

content/tutorials/generic/I2s-library-examples/I2s-library-examples.md

@@ -150,7 +150,7 @@ const int amplitude = 500; // amplitude of square wave
 
 const int sampleRate = 8000; // sample rate in Hz
 
-const int halfWavelength = (sampleRate / frequency); // half wavelength of square wave
+const int halfWavelength = (sampleRate / frequency) / 2; // half wavelength of square wave
 
 short sample = amplitude; // current sample value
 int count = 0;
@@ -192,4 +192,4 @@ void loop() {
 
   count++;
 }
-```
+```