The ornamental design for a keyboard, as shown and described.
FIG. 1 is a front, bottom, right-side perspective view of a keyboard in accordance with our new design;
FIG. 2 is a bottom plan view of the keyboard shown in FIG. 1;
FIG. 3 is a top plan view of the keyboard shown in FIG. 1;
FIG. 4 is a left-side elevational view of the keyboard shown in FIG. 1;
FIG. 5 is a right-side elevational view of the keyboard shown in FIG. 1;
FIG. 6 is a front elevational view of the keyboard shown in FIG. 1; and,
FIG. 7 is a rear elevational view of the keyboard shown in FIG. 1.
The broken lines immediately adjacent the shaded area represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.