The ornamental design for a keyboard, as shown and described.
FIG. 1 is a front perspective view of a keyboard showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom view thereof.
The long-dash-short dash lines surrounding the set of ten unclaimed keys immediately adjacent to the right and to the left of the split in FIGS. 1 and 2 indicate the bounds of the claimed design. While all other broken lines are for environmental purposes only and forms no part of the claimed design.