doc: west: Update outdated link to SPDX specification
doc: west: Update outdated link to SPDX specification

This commit updates the outdated link to the SPDX specification.

Signed-off-by: Stephanos Ioannidis <d862a8ba07ce1656f326514118a4618b42d2d2c0@nordicsemi.no>

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