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 front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a front perspective view thereof, shown together with a representative tablet computer, in which the broken line showing of the table computer is for the purpose of illustrating environmental structure and forms no part of the claimed design; and,
FIG. 9 is a left side elevational view thereof, in which the phantom line showing of the key board in an alternate position is for illustrative purposes and forms no part of claimed design.