The ornamental design for a ring binder, as shown and described.
FIG. 1 is a left side elevational view of the ring binder of my design. The broken lines of the separators attached to the rings form no part of the claimed design. The right side elevational view of the ring binder is a mirror image of the left side; and,
FIG. 2 is a perspective view showing the ring binder of my design in an open position. Only the covers, spine and rings form part of the claimed design.
The top cover, bottom cover and spine of the ring binder are flat and unornamented.