The ornamental design for a shaft, as shown and described.
FIG. 1 is a top left side isometric view of a shaft in accordance with the new design;
FIG. 2 is a left side elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a front isometric view thereof; and,
FIG. 7 is a rear isometric view thereof.
The broken lines shown in FIGS. 1, 4, and 7 represent the bounds of the claimed design and form no part of the claimed design. The break in FIGS. 1-5 show the shaft in two parts for ease of illustration.