The ornamental design for a keyboard, as shown and described.
FIG. 1 is a front view of a keyboard according to a first embodiment of the claimed design; and,
FIG. 2 is a front view of a keyboard according to a second embodiment of the claimed design.
The broken lines shown in the figures show portions of the keyboard that form no part of the claimed design. The surface of the claimed design is uniform of no appreciable thickness. Other elevational and orthographic views are not disclosed and form no part of the claimed design.