Update serial.rst

Author: earlephilhower

docs/serial.rst

@@ -50,8 +50,8 @@ Arduino `Serial Reference <https://www.arduino.cc/reference/en/language/function
 RP2040 Specific SerialUSB methods
 ---------------------------------
 
-int Serial.ignoreFlowControl(bool ignore)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+void Serial.ignoreFlowControl(bool ignore)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 In some cases, the target application will not assert the DTR virtual line, thus preventing writing operations to succeed.
 
 For this reason, the SerialUSB::ignoreFlowControl() method disables the connection's state verification, enabling the program to write on the port, even though the data might be lost.