The ornamental design for a sofa, as shown and described.
FIG. 1 is a front perspective view of the sofa showing my design;
FIG. 2 is top plan view thereof;
FIG. 3 is a back view thereof; and,
FIG. 4 is a side view thereof, an opposite side view being a mirror image thereof.
The broken lines shown on the sofa are understood to represent stitching. The dotted lines of the center feet elements are for illustrative purposes only and form no part of the claimed design.