FIG. 1 is a top left front perspective view of a refrigerator showing our new design; FIG. 2 is a front view thereof; FIG. 3 is a right-side elevation view thereof; FIG. 4 is a left-side elevation view thereof; FIG. 5 is a top plan view thereof; and, FIG. 6 is a bottom plan view thereof. The dashed broken lines are for environmental purposes only and form no part of the claimed design.