The ornamental design for a checkerboard, as shown and described.
FIG. 1 is a plan view of the checker board of the present invention, the
shading designating color, the solid black representing the color black;
the lines angled upward to the right representing the color brown, the
narrowly spaced lines angled upward to the left representing the color
green; and the wider spaced lines angled upward to the left representing
the color light green.