doc: boards: arm: nrf52840_pca10059: fix instructions for flashing
doc: boards: arm: nrf52840_pca10059: fix instructions for flashing

The instructions for flashing the board using nrfutil were incorrect.

Signed-off-by: Emanuele Di Santo <58b3ebaa556fff9ee88aab1044a35f0af0dd7061@nordicsemi.no>

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