The ornamental design for a keyboard, as shown and described.
1. Keyboard
1.1: Top, rear and left side perspective view
1.2: Top, rear and right side perspective view
1.3: Rear plan view
Drawings 1.1to 1.3depict an ornamental design for a keyboard; the evenly dotted broken lines in drawings 1.1to 1.3illustrate unclaimed features of a keyboard and form no part of the claimed design.