doc: device_mgmt: smp: Add slot number to erase command
doc: device_mgmt: smp: Add slot number to erase command

A slot number can now be provided to the mcumgr erase command to erase
a specific slot instead of slot 1, this documents the change in the
SMP group 1 documentation page.

Signed-off-by: Jamie McCrae <7c977670495befe33b67661379bf81cc6b3660ab@nordicsemi.no>

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