Fix definition for UART3

Author: facchinm

cores/arduino/Serial.cpp

@@ -128,9 +128,9 @@ UART UART2(SERIAL2_TX, SERIAL2_RX, NC, NC);
 #if SERIAL_HOWMANY > 2
 
 #ifdef SERIAL3_RTS
-UART UART1(SERIAL3_TX, SERIAL3_RX, SERIAL3_RTS, SERIAL3_CTS);
+UART UART3(SERIAL3_TX, SERIAL3_RX, SERIAL3_RTS, SERIAL3_CTS);
 #else
-UART UART1(SERIAL3_TX, SERIAL3_RX, NC, NC);
+UART UART3(SERIAL3_TX, SERIAL3_RX, NC, NC);
 #endif
 
 #endif