The ornamental design for prosthesis, as shown and described.
FIG. 1 is a top perspective view;
FIG. 2 is a front view;
FIG. 3 is a back view;
FIG. 4 is a right side view;
FIG. 5 is a left side view;
FIG. 6 is a top view; and,
FIG. 7 is a bottom view.
The broken line showing of the holes and bottom surface is for illustrative purpose only and forms no part of the claimed design.