The ornamental design for a keyboard, as shown and described.
FIG. 1 is a perspective view of the keyboard of the present invention wherein the broken lines are shown for environment only and form no part of the claimed design;
FIG. 2 is a top, front, and side perspective view thereof shown partially opened;
FIG. 3 is a top, front and side perspective view thereof shown in the closed position;
CORRECTIVE ASSIGNMENT TO CORRECT THE ASSIGNOR PREVIOUSLY RECORDED ON REEL 051979 FRAME 0165 ASSIGNOR S HEREBY CONFIRMS THE ASSIGNMENT OFASSIGNORS INTEREST