stm32f4: fix minor typo in comment

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
To: u-boot@lists.denx.de
To: Kamil Lulko <rev13@wp.pl>
Cc: Tom Rini <trini@konsulko.com>

include/configs/stm32f429-discovery.h

@@ -52,7 +52,7 @@
 #define CONFIG_STM32_SERIAL
 /*
  * Configuration of the USART
- * 1:   TX:PA9  PX:PA10
+ * 1:   TX:PA9  RX:PA10
  * 2:   TX:PD5  RX:PD6
  * 3:   TX:PC10 RX:PC11
  * 6:   TX:PC6  RX:PC7