Specify that the MicroSD card should be formatted as MBR and not GPT

I followed the 05 steps:

https://github.com/rust-embedded/rust-raspberrypi-OS-tutorials/blame/644474cc09f755249f9c55d99a5d1e07a2562fc7/05_drivers_gpio_uart/README.md#L83

But I formatted it as GPT. It did not work (Rpi 3b). After repeating the steps but formatted as MBR, it worked.