Fix Debounce example to work as described

This alters the debounce example code to toggle the LED rather than just use
the button state to set the LED state. The comments and description seem to imply that this is the intended behaviour.

This came up on reddit today: http://www.reddit.com/r/arduino/comments/15ko75/arduino_tutorial_2_digital_debounce_problem and I also noticed a previous bug referencing the same issue (#293).