I claim the ornamental design for the American flag ball, as shown and described.
FIG. 1 is a front view of the flag ball;
FIG. 2 is cropped bottom view thereof, wherein the cropped portions are indicated by the dashed lines;
FIG. 3 is a right view thereof;
FIG. 4 is a top front and left perspective view thereof;
FIG. 5 is a front and right view thereof; and,
FIG. 6 is a top view thereof.
The broken lines shown in FIG. 1 depict a portion of the American flag ball that forms no part of the claimed design, while the circular broken line in FIG. 2 defines the bounds of the claim within the bottom surface.