The ornamental design for an exercise apparatus, as shown and described.
FIG. 1 is the front perspective view of the exercise apparatus;
FIG. 2 is the right side view of the exercise apparatus;
FIG. 3 is the top view of the exercise apparatus;
FIG. 4 is the front view of the exercise apparatus; and,
FIG. 5 is the rear view of the exercise apparatus.
Broken lines, where utilized, are for illustrative purposes only and are not intended to limit the claimed design to the features shown in broken lines.