samples: mgmt: updatehub: add label directive
samples: mgmt: updatehub: add label directive

This sample was missing a label to place it in generated documentation.

Signed-off-by: Peter Bigot <a4112a160b558d5427af084318038e19a95979ee@nordicsemi.no>

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