Bugfix: PitchDetection returning the wrong pitch (#187)

When converted to the current syntax, the operation was the wrong way around.
See https://github.com/marl/crepe/blob/gh-pages/crepe.js#L141 for the original.

Author: meiamsome

src/PitchDetection/index.js

@@ -79,7 +79,7 @@ class PitchDetection {
         const productSum = products.dataSync().reduce((a, b) => a + b, 0);
         const weightSum = weights.dataSync().reduce((a, b) => a + b, 0);
         const predictedCent = productSum / weightSum;
-        const predictedHz = 10 * ((predictedCent / 1200.0) ** 2);
+        const predictedHz = 10 * (2 ** (predictedCent / 1200.0));
 
         const frequency = (confidence > 0.5) ? predictedHz : null;
         this.frequency = frequency;