fix start4.elf file naming to boot on Raspi 4

06_drivers_gpio_uart/README.md

@@ -53,7 +53,7 @@ init_uart_clock=48000000
 
 3. Copy the following files from the [Raspberry Pi firmware repo](https://github.com/raspberrypi/firmware/tree/master/boot)  onto the SD card:
     - [fixup4.dat](https://github.com/raspberrypi/firmware/raw/master/boot/fixup4.dat)
-    - [start4.elf](https://github.com/raspberrypi/firmware/raw/master/boot/start.elf)
+    - [start4.elf](https://github.com/raspberrypi/firmware/raw/master/boot/start4.elf) (on the card, the file must be named `start.elf`, without the 4)
     - [bcm2711-rpi-4-b.dtb](https://github.com/raspberrypi/firmware/raw/master/boot/bcm2711-rpi-4-b.dtb)
 4. Run `BSP=rpi4 make` and copy the [kernel8.img](kernel8.img) onto the SD card.