The ornamental design for a platter, as shown and described.
FIG. 1 is a front, perspective view of a platter showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof, the left side view being a mirror image to the right side view;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view.
The features shown in broken lines portions of the platter forming no part of the claimed design. The features shown in long dash-short dash bolded broken lines indicate the bounds of the claimed design and otherwise form no part of the claimed design.