boards/feather-nrf52840: fix shell command formatting in doc
boards/feather-nrf52840: fix shell command formatting in doc

OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,authmillenon/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ant9000/RIOT,OlegHahm/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,authmillenon/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,ant9000/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,miri64/RIOT