Update set-map.md

修复了变量未定义报错

docs/set-map.md

@@ -13,7 +13,7 @@ var s = new Set();
 
 [2,3,5,4,5,2,2].map(x => s.add(x))
 
-for (i of s) {console.log(i)}
+for (let i of s) {console.log(i)}
 // 2 3 5 4
 ```