The ornamental design for a tie down, as shown and described.
FIG. 1 is a side view of the design for a tie down, with the ring member thereof shown in the partially extended position;
FIG. 2 is a top view of the tie down of FIG. 1;
FIG. 3 is a reduced-scale side perspective view of the tie down of FIG. member thereof shown in the closed position, with the ring member thereof shown in the closed position;
FIG. 4 is a side view of the tie down of FIG. 3;
FIG. 5 is a top view of the tie down of FIG. 3;
FIG. 6 is a perspective view of the tie down of FIG. 1, with the ring member thereof shown in the fully extended position;
FIG. 7 is a side view of the tie down of FIG. 6; and,