The ornamental design for a rotating groundpronganchor, as shown and described.
FIG. 1 is a rear perspective view of a rotating ground prong anchor showing our new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken lines immediately adjacent to and surrounding the shaded areas represent the bounds of the claimed design and form no part thereof. All other broken lines represent the portions of the rotating ground prong anchor that form no part of the claimed design.