The ornamental design for a sofa, as shown and described.
FIG. 1 is a perspective view of a sofa, showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The cross-hatching shading in the figures is fragmentarily shown for convenience of illustration, it being understood that the cross-hatching is uniformly continued over the surface on which it appears. Features shown in broken lines in FIGS. 1-7 illustrate portions of the sofa that form no part of the claimed design.