The ornamental design for a keyboard, as shown and described.
FIG. 1 is a perspective view of a keyboard showing my new design wherein the power to the keyboard is turned on; and
FIG. 2 is a top plan view thereof.
FIG. 3 is a perspective view of another embodiment of a keyboard showing my new design wherein the power to the keyboard is turned on; and,
FIG. 4 is a top plan view thereof.
The dot-dash broken lines shown in FIGS. 1-4 that are immediately adjacent to the solid lines and shaded areas represent the bounds of the claimed design, while all other lines are directed to environment and form no part of the claimed design.