The ornamental design for a sofa, as shown and described.
FIG. 1 is a front perspective view of the sofa of my design;
FIG. 2 is top plan view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a side view thereof; and,
FIG. 5 is an opposite side view thereof.
The broken lines shown on the sofa are understood to represent stitching. The dotted lines of the elements in FIG. 5 are for illustrative purposes only and form no part of the claimed design.