board/stmicroelectronics/stm32mp135f-dk: fix boot steps in readme

The instructions in the readme use incorrect connector names.

See also the official devkit documentation:
https://wiki.st.com/stm32mpu/wiki/Getting_started/STM32MP1_boards/STM32MP135x-DK/Let%27s_start/Unpack_the_STM32MP135x-DK_board#Out-of-the-box_kit

Also the board does not show the console on the screen.

This may have been a copy & paste oversight from the STM32MP157 readme.

Signed-off-by: Gero Schwäricke <gero.schwaericke@sevenlab.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/stmicroelectronics/stm32mp135f-dk/readme.txt

@@ -27,12 +27,11 @@ Copy the bootable "sdcard.img" onto an microSD card with "dd":
 Boot the board
 ==============
 
- (1) Insert the microSD card in connector CN15
+ (1) Insert the microSD card in connector CN3
 
- (2) Plug a micro-USB cable in connector CN11 and run your serial
+ (2) Plug a micro-USB cable in connector CN10 and run your serial
      communication program on /dev/ttyACM0.
 
- (3) Plug a USB-C cable in CN6 to power-up the board.
+ (3) Plug a USB-C cable in CN12 to power-up the board.
 
- (4) The system will start, with the console on UART, but also visible
-     on the screen.
+ (4) The system will start, with the console on UART.