The ornamental design for a doll, as shown and described.
FIG. 1 is a perspective view of a doll showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view, the opposite side being substantially the same;
FIG. 5 is a top plan view; and
FIG. 6 is a bottom plan view.