在字符串指定的索引下添加字符，输出换行

 static void Main(string[] args)
        {
            string ExecuteDesc = "所要插入的字符串";
            int count = ExecuteDesc.Length / 20;
            for (int j = 1; j <= count; j++)
            {
                ExecuteDesc = j == 1 ? ExecuteDesc.Insert(j * 20, "<br/>") : ExecuteDesc.Insert(j * 20 + (j - 1) * 5, "<br/>");
            }
        }