🐛 Fix Y_SERIAL_RX_PIN for FYSETC S6 (#23055)

Marlin/src/pins/stm32f4/pins_FYSETC_S6.h

     #define Y_SERIAL_TX_PIN                 PE14
   #endif
   #ifndef Y_SERIAL_RX_PIN
-    #define Y_SERIAL_RX_PIN                 PE13
+    #define Y_SERIAL_RX_PIN                 PC4
   #endif
   #ifndef Z_SERIAL_TX_PIN
     #define Z_SERIAL_TX_PIN                 PD11