The ornamental design for a keyboard, as shown and described.
FIG. 1 is a perspective view of a keyboard showing the new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a first side view thereof;
FIG. 5 is a second side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line circles on the fourth and seventh keys in the center row of FIGS. 1 and 6 form no part of the claimed design. The broken line across the front portion of the design in FIGS. 1, 2, 4, and 5 forms no part of the claimed design. The non-shaded regions, and the broken lines within the non-shaded regions, in FIGS. 3–7, are for the purpose of illustrating portions of the article and form no part of the claimed design.