net: net_addr_ntop: Clarify docstring
net: net_addr_ntop: Clarify docstring

The string returned is definitely NUL terminated, previously was
apaprently a typo.

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,kraj/zephyr,explora26/zephyr,galak/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,punitvara/zephyr,finikorg/zephyr,finikorg/zephyr,ldts/zephyr,kraj/zephyr,kraj/zephyr,explora26/zephyr,kraj/zephyr,ldts/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,explora26/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,nashif/zephyr