Minor docs change on Chapter 2

[archermarx]

There was a typo in Chapter 2 in the part about blocking signals which has been fixed. The section:

    connect_signal_clicked!(button_02) do button_02::Button, button_01::Button
        println("02 clicked")

        # block self (02)
        set_signal_clicked_blocked!(button_02, true)

        # activate other (01)
        emit_signal_clicked(button_01)

        # unblock self (02)
        set_signal_clicked_blocked!(button_02, false)
    end

should have had a button_01 as a second arg to connect_signal_clicked!