The ornamental design for sofa, as shown and described.
FIG. 1 is a front perspective view of a first embodiment of a sofa having an armed end unit, a straight intermediate unit and a chaise-type end unit showing my new design;
FIG. 2 is a front perspective view of a second embodiment of a sofa showing my new design;
FIG. 3 is a side view of the second embodiment of a sofa showing my new design; and,
FIG. 4 is a front view of the second embodiment of a sofa showing my new design.