fix naming

snippets/flatten.md

@@ -0,0 +1,8 @@
+### Flatten array
+
+Use `Array.reduce()` to get all elements inside the array and `concat()` to flatten them.
+
+```js
+const flatten = arr => arr.reduce((a, v) => a.concat(v), []);
+// flatten([1,[2],3,4]) -> [1,2,3,4]
+```