Update 0203.移除链表元素.md

Creating a dummy head variable makes the logic clear.

problems/0203.移除链表元素.md

@@ -342,14 +342,14 @@ function removeElements(head: ListNode | null, val: number): ListNode | null {
 
 ```typescript
 function removeElements(head: ListNode | null, val: number): ListNode | null {
-    head = new ListNode(0, head);
-    let pre: ListNode = head, cur: ListNode = head.next;
+    let dummyHead = new ListNode(0, head);
+    let pre: ListNode = dummyHead, cur: ListNode | null = dummyHead.next;
     // 删除非头部节点
     while (cur) {
         if (cur.val === val) {
             pre.next = cur.next;
         } else {
-            pre = pre.next;
+            pre = cur;
         }
         cur = cur.next;
     }