fix: 修复C#代码高亮问题

problems/0106.从中序与后序遍历序列构造二叉树.md

@@ -1229,7 +1229,7 @@ impl Solution {
 }
 ```
 ### C#
-```C#
+```csharp
 public TreeNode BuildTree(int[] inorder, int[] postorder)
 {
     if (inorder.Length == 0 || postorder.Length == null) return null;