The ornamental design for a down marker, as shown and described.
FIG. 1 is a front perspective view of a down marker showing our new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines illustrate environment and forms no part of the claimed design. The break lines indicate undetermined length of the vertical stand and portions between said break lines form no part of the claimed design.