The ornamental design for a corkscrew, as shown and described.
FIG. 1 is a perspective view of a corkscrew according to our new design;
FIG. 2 is a left side elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a front elevation view thereof;
FIG. 5 is a rear elevation view thereof;
FIG. 6 is a top elevation view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dash-dot-dash broken lines illustrate a boundary line of the claimed design. The dash-dash broken lines showing in the views illustrate unclaimed portions of the corkscrew and form no part of the claimed design.