The ornamental design for an exercise apparatus, as shown and described.
FIG. 1 is a perspective view of an exercise apparatus showing our new design, shown in an open position;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view thereof, shown in a closed position.
The broken lines shown in the figures are for the purpose of illustrating portions of the exercise apparatus, which form no part of the claimed design.