The ornamental design for a knife, as shown and described.
FIG. 1 is a top perspective view of a knife;
FIG. 2 is a top view thereof; and,
FIG. 3 is a bottom view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the knife that form no part of the claimed design. Portions of the knife not illustrated including the right side view and left side view are disclaimed.