doc: win: Invoke pip3 instead of pip to be safe
doc: win: Invoke pip3 instead of pip to be safe

It has been observed that users might "default" to Python2 and pip
will therefore install packages for the wrong Python. pip3 appears to
always be installed when Python3 is installed, so we invoke pip3
instead of pip2 to be safe.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

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