I claim the ornamental design for a toothbrush with a curved head, as shown and described.
FIG. 1 is a front view of a toothbrush with a curved head.
FIG. 2 is a right side view of the toothbrush with a curved head.
FIG. 3 is a rear view of the toothbrush with a curved head.
FIG. 4 is a left side view of the toothbrush with a curved head.
FIG. 5 is a perspective view of the toothbrush with a curved head.
FIG. 6 is a bottom view of the toothbrush with a curved head; and,
FIG. 7 is a top view of the toothbrush with a curved head.
The portions of the toothbrush with a curved head shown with broken lines in the drawing figures are features of the article of manufacture that form no part of the claimed design.