The ornamental design for a keyboard, as shown and described.
FIG. 1 is a front perspective view of a keyboard showing our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a front perspective view of an enlarged region of the keyboard containing the claimed region; and,
FIG. 5 is a rear perspective view thereof.
The broken lines appearing along the sides and in front of the four claimed keys are understood to define the bounds of the claimed design, and the broken line showing of the remainder of the keyboard is for environmental purposes only and forms no part of the claimed design.