The ornamental design for a keyboard, as shown and described.
FIG. 1 is a perspective view of the keyboard, showing an example of the keyboard as part of an exemplary notebook computer; and,
FIG. 2 is a top view of the keyboard, showing an example of the keyboard as part of an exemplary notebook computer.
The broken lines shown surrounding the keys in the figures represent the bounds of the claimed design, while all other lines are directed to environment and form no part of the claimed design.