boards: mps3_an547: Update obsolete emulation instructions
boards: mps3_an547: Update obsolete emulation instructions

The commit 94428044e2addec0bb631d33821cd0c6086bc243, which introduced
this behavioural change, forgot to update the instructions provided in
the comments.

This commit updates the obsolete instructions for using an alternate
emulation platform in the comments.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

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