I claim the ornamental design for food grater, as shown and described.
FIG. 1 is a front, top perspective view of the food grater embodying the new design;
FIG. 2 is a right side view thereof, the left side being a mirror image thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a front view thereof; and,
FIG. 6 is a rear view thereof.
FIGS. 1-6 show features of a food grater in phantom lines. These features are shown for environmental purposes and do not form any part of the claimed design. Further, the phantom lines immediately adjacent the solid-line portions of the design form the boundary of the design, with the phantom lines forming no part of the claimed design.