Add BMI function using proper formula and one-decimal formatting

Author: ChinweP

Sprint-2/3-mandatory-implement/1-bmi.js

@@ -16,4 +16,6 @@
 
 function calculateBMI(weight, height) {
     // return the BMI of someone based off their weight and height
+    const bmi = (weight / height **2);
+    return bmi.toFixed(1);
 }