I claim the ornamental design for a head of a toothbrush, as shown and described.
FIG. 1 is a first perspective view of a head of a toothbrush according to my new design;
FIG. 2 is an enlarged bottom view thereof;
FIG. 3 is an enlarged top view thereof;
FIG. 4 is an enlarged left side view thereof;
FIG. 5 is an enlarged right side view thereof;
FIG. 6 is an enlarged inner end view thereof;
FIG. 7 is an enlarged outer end view thereof;
FIG. 8 is an enlarged bottom perspective view thereof; and,
FIG. 9 is an enlarged top perspective view thereof.
The broken line showing of a handle in FIG. 1 is included for the purpose of illustrating environmental structure that forms no part of the claimed design.