The ornamental design for an eraser, as shown and described.
FIG. 1 is a top and side perspective view of an eraser, showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a side elevational view thereof taken from the left of FIG. 5; the side opposite to that shown in FIG. 4 being the mirror image thereof; and,
FIG. 5 is a side elevational view thereof taken from the left of FIG. 3; the side opposite to that shown in FIG. 5 being the mirror image thereof.