Update README.md

Geekhyt · web-flow · commit 53af703e6b9e · 2021-02-06T15:19:36.000+08:00
diff --git a/README.md b/README.md
@@ -42,3 +42,6 @@
 - [55. 跳跃游戏](https://github.com/Geekhyt/javascript-leetcode/issues/24)
 - [455. 分发饼干](https://github.com/Geekhyt/javascript-leetcode/issues/25)
 - [860. 柠檬水找零](https://github.com/Geekhyt/javascript-leetcode/issues/26)
+
+## 回溯
+- [17. 电话号码的字母组合](https://github.com/Geekhyt/javascript-leetcode/issues/27)
