The ornamental design for a dental floss, as shown and described.
FIG. 1 is a perspective view of a dental floss design showing my new design.
FIG. 2 is a front view of a dental floss.
FIG. 3 is a rear view of a dental floss.
FIG. 4 is a left side view of a dental floss.
FIG. 5 is a right side view of a dental floss.
FIG. 6 is a top view of a dental floss; and,
FIG. 7 is a bottom view of a dental floss.
Any shading and cross-hatching are not features of the design but are utilized to illustrate the surface contours of the dental floss design in the drawings.