doc: nrf52840dongle_nrf52840: fix build instructions for external probes
doc: nrf52840dongle_nrf52840: fix build instructions for external probes

In order for the application to be flashed beginning at address 0,
which is desired when flashing with an external probe,
BOARD_HAS_NRF5_BOOTLOADER must be set to n.

Signed-off-by: Jacob Siverskog <f862f167b85d41b225785c70d70808bc7337c1fe@teenage.engineering>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr