The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a perspective view showing my new design:
FIG. 2 is a top plan view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines showing the brushing structures on the heads, and the broken lines showing the small circular elements on the handle, illustrate portions of the toothbrush that form no part of the claimed design. The small circular elements are raised, as shown in FIGS. 4 and 5.