private void dataGridView2_CellContentClick(object sender, DataGridViewCellEventArgs e)
        {
            SqlConnection con = new SqlConnection(polaczenie_baza);
            dataGridView2.DataSource = null;
            dataGridView2.Update();
            dataGridView2.Refresh();
            SqlCommand command1 = new SqlCommand("select id, data_przychodu, rodzaj, kwota from przychody", con);
            con.Open();
            command1.CommandType = CommandType.Text;
            SqlDataAdapter sqlDataAdap = new SqlDataAdapter(command1);
            DataTable bookings = new DataTable();
            sqlDataAdap.Fill(bookings);
            dataGridView2.DataSource = bookings;
            con.Close();
        }