The ornamental design for a putterhead, as shown and described.
FIG. 1 is a top view of the putter head, showing my new design.
FIG. 2 is a front view of the putter head.
FIG. 3 is a heel view of the putter head; and,
FIG. 4 is a bottom view of the putter head.
The broken line showing a hole for a shaft and weighting on the bottom of the putter head is for illustrative purposes only and forms no part of the claimed invention.