Add a simple example for sum

Author: LarryLuTW

snippets/sum.md

@@ -9,5 +9,6 @@ const sum = (...arr) => [...arr].reduce((acc, val) => acc + val, 0);
 ```
 
 ```js
+sum(1, 2, 3, 4); // 10
 sum(...[1, 2, 3, 4]); // 10
 ```