Fix broken link in documentation

Fix link in docs/firmware-update.md and docs/change-log.md:
https://github.com/ARM-software/arm-trusted-firmware/wiki/ARM-Trusted-Firmware-Image-Terminology



Change-Id: I2d51d373fd0f7da59b548cd6bed52c47772014fd
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>