The ornamental design for a keyboard, as shown and described.
FIG. 1 is a front, bottom, right perspective view of a keyboard showing my new design;
FIG. 2 is a rear, bottom, left perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The portions that are shown as broken lines are for illustrative purposes only and form no part of the claimed design. The dot-dash broken lines define the boundaries of, but are not included in, the claimed design.