May want to update docs.

I had to dig around in the .cpp file to figure out that a button is initialized via 
"Button button = Button(3, BUTTON_PULLDOWN)"
rather than 
"Button button = Button(3, PULLDOWN)"
as the documentation says.

I hate it when this happens, :( now I have to test every function in the library to make sure that it actually works as it should, due to not being able to trust the documentation.
