I claim the ornamental design for a bolt head, as shown and described.
FIG. 1 is a perspective view of a bolt head, showing my new design;
FIG. 2 is a top plan view of the invention; and,
FIG. 3 is a left-side elevational view of the invention, the right-side elevational view, front elevational view, and rear elevational view being a mirror image thereof.
The broken lines showing a bolt head perimeter and threads in FIGS. 1, 2, and 3 are included for the purpose of illustrating environment and form no part of the claimed design.