The ornamental design for a knife, as shown and described.
FIG. 1 is a right side perspective view of a knife according to the present invention.
FIG. 2 is a right side view of the knife according to the present invention.
FIG. 3 is a left side view of the knife according to the present invention.
FIG. 4 is a top view of the knife according to the present invention.
FIG. 5 is a bottom view of the knife according to the present invention.
FIG. 6 is a back view according to the present invention; and,
FIG. 7 is a front view of a knife according to the present invention.
The broken lines showing a blade and pins in the handle show structure that is not part of the claimed design, but rather show the environment in which the design is embodied or applied to that is not considered part of the claimed design.