Snippet format update

To match the starter (for the migration)

snippets/flatten.md

@@ -1,4 +1,7 @@
-### flatten
+---
+title: flatten
+tags: array,intermediate
+---
 
 Flattens an array up to the specified depth.
 
@@ -15,4 +18,4 @@ const flatten = (arr, depth = 1) =>
 ```js
 flatten([1, [2], 3, 4]); // [1, 2, 3, 4]
 flatten([1, [2, [3, [4, 5], 6], 7], 8], 2); // [1, 2, 3, [4, 5], 6, 7, 8]
-```
+```