changed the pin where to connect the button from 2 to 4 to be compliant with the comment in the header and the tutorial.

Fixes #1396

Author: Fede85

build/shared/examples/09.USB/Keyboard/KeyboardMessage/KeyboardMessage.ino

@@ -18,7 +18,7 @@
  http://www.arduino.cc/en/Tutorial/KeyboardButton
  */
 
-const int buttonPin = 2;          // input pin for pushbutton
+const int buttonPin = 4;          // input pin for pushbutton
 int previousButtonState = HIGH;   // for checking the state of a pushButton
 int counter = 0;                  // button push counter