The ornamental design for a sofa, as shown and described.
FIG. 1 is a front perspective view of a sofa embodying the new design;
FIG. 2 is a close-up side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
FIGS. 1-8 show portions of an article of furniture in broken lines. The portions shown in broken lines are for environmental purposes only and form no part of the claimed design. Stipple shading is added to convey surface contouring; the stipple shading is not to be understood as conveying a particular surface finish.