Update standard-deviation.md

snippets/standard-deviation.md

@@ -9,7 +9,7 @@ Since there are two types of standard deviation, population and sample, you can
 const standardDeviation = (arr, usePopulation) => {
   const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
   return Math.sqrt(
-    arr.reduce((acc, val) => acc.concat(Math.pow(val - mean / arr.length, 2)), [])
+    arr.reduce((acc, val) => acc.concat(Math.pow(val - mean, 2)), [])
        .reduce((acc, val) => acc + val, 0) 
     / (arr.length - (usePopulation ? 0 : 1))
   );