Update flatten-array.md

add closing bracket to the comment part

snippets/flatten-array.md

@@ -4,5 +4,5 @@ Use `reduce()` to get all elements inside the array and `concat()` to flatten th
 
 ```js
 const flatten = arr => arr.reduce( (a, v) => a.concat(v), []);
-// flatten([1,[2],3,4) -> [1,2,3,4]
+// flatten([1,[2],3,4]) -> [1,2,3,4]
 ```