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 top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a front view thereof; and,
FIG. 7 is a rear view thereof.
The broken line showing of the remainder of the keyboard and the unshaded regions therein in FIGS. 1-7 is for environmental purposes only and forms no part of the claimed design.