fix: 修复C#代码高亮问题

problems/0654.最大二叉树.md

@@ -583,7 +583,7 @@ impl Solution {
 }
 ```
 ### C#
-```C#
+```csharp
 public TreeNode ConstructMaximumBinaryTree(int[] nums)
 {
     if (nums.Length == 0) return null;