The ornamental design for a corkscrew, as shown and described.
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FIG. 1 is a front elevation view of a corkscrew, according to my new design; the rear elevation view thereof being identical thereto;
FIG. 2 is a side elevation view thereof; the opposite side elevation view thereof being identical thereto; and,
FIG. 3 is a top plan view thereof.
The broken lines depict environmental subject matter only and form no part of the claimed design.