I claim the ornamental design for a flag pole cap, as shown and described.
FIG. 1 is a perspective view of the flag pole cap showing my new design including a symmetrical base with a beveled upper perimeter corner and a central cylindrical bore and four cone-like horns spaced at equal intervals along the perimeter, each horn extending symmetrically upwardly and outwardly from the upper corner of the perimeter to its respective distal tip.
FIG. 2 is a front elevation view of my new design, the rear and left and right elevation views being identical thereto.