fix: 修复C#代码高亮问题

problems/面试题02.07.链表相交.md

@@ -503,7 +503,7 @@ object Solution {
 }
 ```
 ### C#
-```C#
+```csharp
 public ListNode GetIntersectionNode(ListNode headA, ListNode headB)
 {
     if (headA == null || headB == null) return null;