The ornamental design for a keyboard, as shown and described.
FIG. 1 is a perspective view of a keyboard showing my new design;
FIG. 2 is another 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;
FIG. 8 is a bottom plan view thereof;
FIG. 9 is an enlarged view of portion 9 shown in FIG. 2; and,
FIG. 10 is an enlarged view of portion 10 shown in FIG. 8.
The evenly dashed broken lines shown in the drawings depict portions of the keyboard that form no part of the claimed design. The unevenly spaced broken lines shown in FIGS. 2 and 8-10 are used for defining areas of FIGS. 2 and 8 that are shown enlarged in FIGS. 9 and 10.