samples: psa-firmware: Correct readme instructions
samples: psa-firmware: Correct readme instructions

Prior instructions were incomplete, included extra, unneeded ways to
build the app.

This should simplify the instructions.

Signed-off-by: Jimmy Brisson <de33656aa60791325e53378be66c520c3736548a@linaro.org>

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