The ornamental design for a knob, as shown and described.
FIG. 1 is a top, left, and rear isometric view of a knob according to the present design;
FIG. 2 is a top plan view of the knob of FIG. 1;
FIG. 3 is a right side elevational view of the knob of FIG. 1;
FIG. 4 is a left side elevational view of the knob of FIG. 1; and,
FIG. 5 is a front elevational view of the knob of FIG. 1.
The dash-dash-dash lines are included for the purpose of illustrating portions that form no part of the claimed design. The dash-dot-dash broken lines depict boundary lines and form no part of the claimed design.