doc/reference/runtime_conf: improve example snippet
doc/reference/runtime_conf: improve example snippet

Improve settings example snippet so that it builds and works as
expected. For usability purposes, add print of stored value.

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

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