The ornamental design for an ornament, as shown and described.
FIG. 1 is a top perspective view of an ornament showing our 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 top perspective view thereof, shown in a used condition with a rope and tree shown in broken lines that form no part of the claimed design.