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 line immediately adjacent to and surrounding the shaded area represents the bounds of the claimed design and forms no part thereof. The broken lines depicting the buttons, port, and remainder of the rotating ground prong anchor form no part of the claimed design.