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 left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of various regions in the Figures and the unshaded areas therein is for illustrative purposes only and forms no part of the claimed design.