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>

boards/arm/stm32f412g_disco/stm32f412g_disco.dts

@@ -35,7 +35,7 @@
 			label = "User LD3";
 		};
 		blue_led_4: led_4 {
-			gpios = <&gpioe 4 GPIO_ACTIVE_HIGH>;
+			gpios = <&gpioe 3 GPIO_ACTIVE_HIGH>;
 			label = "User LD4";
 		};
 	};