The ornamental design for a sofa, as shown and described.
FIG. 1 is perspective view of the sofa, illustrating my new design;
FIG. 2 is front elevational view of the sofa;
FIG. 3 is a side elevational view of one side of the sofa, with the side elevational view of the other side of the sofa being a mirror image thereof; and,
FIG. 4 is a top plan view of the sofa.
It is to be understood that the rattan patterning has texture and dimensionality.