I claim the ornamental design for a knob, as shown and described.
FIG. 1 is a top perspective view of a knob;
FIG. 2 is a top plan view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a front elevation view thereof; and,
FIG. 6 is a rear elevation view thereof.
The broken lines are for the purpose of illustrating boundaries that form no part of the claimed design. No bottom view is shown, and any bottom view forms no part of the claimed design.