I claim 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 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;
FIG. 6 is a rear elevational view of FIG. 1;
FIG. 7 is an enlarged top plan view of a left side portion of the keyboard, a central and right side omitted for clarity of illustration; and,
FIG. 8 is an enlarged top plan view of the central portion of the keyboard, the left and right side portions omitted for clarity of illustration.
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 shading lines in the figures depict the colors green, red, blue and purple, it being understood only the key indicia is colored.