I claim the ornamental design for a keyboard, as shown and described.
FIG. 1 is a perspective view of a keyboard, showing a first embodiment of my new design;
FIG. 2 is a right-side elevational view of FIG. 1;
FIG. 3 is a left-side elevational view of FIG. 1;
FIG. 4 is a top plan view of FIG. 1;
FIG. 5 is a front elevational view of FIG. 1; and,
FIG. 6 is a rear elevational view of FIG. 1.
The broken lines showing keys in the figures depict environmental matter and form no part of the claimed design. The bottom plan view forms no part of the claimed design. The shade lines shown on the keys in the figures depict the colors green, red, blue and purple.