The ornamental design for a cone for a river buoy, as shown and described.
FIG. 1 is a perspective view of the cone for the river buoy;
FIG. 2 is a front view of the cone for the river buoy;
FIG. 3 is a rear view of the cone for the river buoy;
FIG. 4 is a left side view of the cone for the river buoy;
FIG. 5 is a right side view of the cone for the river buoy;
FIG. 6 is a top view of the cone for the river buoy;
FIG. 7 is a bottom view of the cone for the river buoy; and,
FIG. 8 is a perspective view of the cone assembled into the river buoy.
The broken lines showing in the figures are included for the purpose of illustrating portions of the cone for the river buoy that form no part of the claimed design.