The ornamental design for a flag, as shown and described.
FIG. 1 is a front view of a flag;
FIG. 2 is a top view of the design of FIG. 1, where the bottom view is a mirror image of the top view;
FIG. 3 is a first side view of the design of FIG. 1; and,
FIG. 4 is a second side view of the design of FIG. 1.
The claimed design is directed to the solid lines and the areas within the solid lines. The broken lines in the drawings represent unclaimed environmental subject matter.