boards: arm: stm32f412g_disco: Fix LED4 pin

On the stm32f412g_disco board, the LED LD4 (blue LED) is actually
on the pin 3 of GPIOE and not on the pin 4

(see https://www.st.com/resource/en/user_manual/um2032-discovery-kit-with-stm32f412zg-mcu-stmicroelectronics.pdf

, page 31)

Signed-off-by: Jacques Supcik <jacques.supcik@hefr.ch>