Deprecated reference to mouse buttons

[NLGS2907]

Dunno if this counts as an issue, but looking through the doc, I realized that it refers to the left, middle and right click buttons of the mouse as 0, 1 and 2 respectively.

python-gamelib/gamelib.py

Line 649 in 9cb0d87

mouse_button: 0, 1 or 2 for left, right and middle mouse buttons respectively.

It may be because they updated it, but Tkinter actually refers to these buttons as 1, 2 or 3 respectively. This could lead to confusion.

Already confirmed this is the case testing it on my own project.