Change all snippets from var to const

snippets/average-of-array-of-numbers.md

@@ -3,6 +3,6 @@
 Use `reduce()` to add each value to an accumulator, initialized with a value of `0`, divide by the `length` of the array.
 
 ```js
-var average = arr =>
+const average = arr =>
   arr.reduce( (acc , val) => acc + val, 0) / arr.length;
 ```