fix: 优化 d.ts

Author: livehigh

index.d.ts

@@ -2624,7 +2624,7 @@ declare class COS {
   /** 获取文件下载链接 @see https://cloud.tencent.com/document/product/436/35651 */
   getObjectUrl(
     params: COS.GetObjectUrlParams,
-    callback: (err: COS.CosError, data: COS.GetObjectUrlResult) => void
+    callback?: (err: COS.CosError, data: COS.GetObjectUrlResult) => void
   ): string;
 
   /** 追加上传 @see https://cloud.tencent.com/document/product/436/7741 */