net: shell: Correct help text for "mem" command
net: shell: Correct help text for "mem" command

Before, it was copy-paste from "iface" command help.

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

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