The ornamental design for a marker, as shown and described.
FIG. 1 is an upper front left perspective view of a marker showing my new design;
FIG. 2 is a front elevation view thereof, with the rear elevation view being a mirror image of the front elevation view;
FIG. 3 is a right side elevation view thereof, with the left side elevation view being a minor image of the right side elevation view;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof, the bottom being a minor image of the top.
The feature shown in broken lines in FIGS. 1, 4, and 5 is included for the purpose of illustrating environment and forms no part of the claimed design.