The ornamental design for a keyboard, as shown and described.
FIG. 1 is a front perspective view of a keyboard showing my new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken line showing of various regions in the figures, including the unshaded areas therein, is for illustrative purposes and forms no part of the claimed design. The difference in shading in the figures indicates a contrast of appearance and does not depict any particular color, texture or material.