The ornamental design of a keyboard, as shown and described.
FIG. 1 is a top plan view of a keyboard according to my new design;
FIG. 2 is a left side perspective view thereof;
FIG. 3 is a right side perspective view thereof; and,
FIG. 4 is another top plan view thereof.
The dashed lines illustrate environment that does not form a part of the present invention, and no claim is made to the material illustrated with dashed lines.
Broken lines shown in the drawings illustrate portions of the keyboard and form no part of the claimed design.