Change all snippets from var to const

snippets/fibonacci-array-generator.md

@@ -4,7 +4,7 @@ Create an empty array of the specific length, initializing the first two values
 Use `reduce()` to add values into the array, using the sum of the last two values, except for the first two.
 
 ```js
-var fibonacci = n =>
+const fibonacci = n =>
   Array.apply(null, [0,1].concat(Array(n-2))).reduce(
     (acc, val, i) => {
       acc.push( i>1 ? acc[i-1]+acc[i-2] : val);