Update 0707.设计链表.md

Format the ListNode definition for java code

Author: hailincai

problems/0707.设计链表.md

@@ -282,12 +282,12 @@ Java：
 ```Java
 //单链表
 class ListNode {
-int val;
-ListNode next;
-ListNode(){}
-ListNode(int val) {
-this.val=val;
-}
+    int val;
+    ListNode next;
+    ListNode(){}
+    ListNode(int val) {
+        this.val=val;
+    }
 }
 class MyLinkedList {
     //size存储链表元素的个数