The ornamental design for a toothbrush, as shown and described.
FIG. 1 is a front perspective view of a toothbrush;
FIG. 2 is a back perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line adjacent the surface shading in FIGS. 1-5 represents the bounds of the claim and forms no part thereof. The broken line showing of a handle illustrates a portion of the toothbrush that forms no part of the claimed design.