boards: mps3_an547: Fix typo in the board documentation
boards: mps3_an547: Fix typo in the board documentation

The mps3_an547 board documentation incorrectly referred to the
Ethos-U55 coprocessor as an FPU (floating point unit) when it is really
an NPU (neural processing unit).

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

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