The ornamental design for an earplug, as shown and described.
FIG. 1 is a front perspective view of an ear plug showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a bottom perspective view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to elements of the article excluded from the claim; the broken lines form no part of the claimed design.