Change all snippets from var to const

snippets/flatten-array.md

@@ -4,6 +4,6 @@ Use recursion.
 Use `reduce()` to get all elements that are not arrays, flatten each element that is an array.
 
 ```js
-var flatten = arr =>
+const flatten = arr =>
   arr.reduce( (a, v) => a.concat( Array.isArray(v) ? flatten(v) : v ), []);
 ```