The ornamental design for a shaft, as shown and described.
FIG. 1 is a top, right, frontal perspective view of a shaft showing my new design according to one embodiment;
FIG. 2 shows portions of the shaft of FIG. 1, enlarged with center section excluded for clarity of illustration;
FIG. 3 is a top view of the shaft of FIG. 1, shown enlarged with center section excluded for clarity of illustration;
FIG. 4 is a frontal view of the shaft of FIG. 1, shown enlarged with center section excluded for clarity of illustration; and,
FIG. 5 is a right end view of the shaft of FIG. 2, the left end being identical, the broken lines showing environment forming no part of the claimed design.