Update the pin modes of flappy_homage.ino

I think BUTTON_PIN needs to be an INPUT pin for this to work, and the pin mode ON_BOARD_LED_PIN is not set to OUTPUT before using it. I tested the proposed change on my Arduino Uno with the MAX7219 and an 8x8 LED matrix and it seems to be working perfectly.

Thank you for making this available, I learned a lot from studying it :)