Correct return type of flush()

Reference:
https://github.com/arduino/ArduinoCore-avr/blob/b7c607663fecc232e598f2c0acf419ceb0b7078c/cores/arduino/Print.h#L90

Author: per1234

Language/Functions/Communication/Stream/streamFlush.adoc

@@ -31,7 +31,7 @@ This function is part of the Stream class, and is called by any class that inher
 
 [float]
 === Returns
-`boolean`
+Nothing
 
 --
 // OVERVIEW SECTION ENDS