The ornamental design for a plectrum, as shown and described.
FIG. 1 is a front perspective view of a plectrum, showing my new design;
FIG. 2 is a front view of the plectrum;
FIG. 3 is a rear view of the plectrum;
FIG. 4 is a left-side view of the plectrum;
FIG. 5 is a right-side view of the plectrum;
FIG. 6 is a top view of the plectrum; and,
FIG. 7 is a bottom view of the plectrum.
The broken line showing is for environmental purposes only and forms no part of the claimed design. The dot-dashed line is intended as a boundary line and forms no part of the claimed design.