The ornamental design for a keyboard, as shown and described.
FIG. 1 is a 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 left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The dashed broken line portions of the keyboard represent unclaimed portions of the claimed design and form no part thereof. The dot-dash broken lines define the bounds of the claimed design and form no part thereof.