We claim the ornamental design for an adjustment handle, as shown and described.
FIG. 1 is a perspective view of an adjustment handle on a stool showing a top, a rear and a right side.
FIG. 2 is another perspective view of the adjustment handle of FIG. 1 showing the top, the rear and a left side.
FIG. 3 is a top plan view.
FIG. 4 is a rear elevation view, the front being a mirror image.
FIG. 5 is a bottom plan view.
FIG. 6 is a left side elevation view; and,
FIG. 7 is a right side elevation view.
The broken line showings of the mounting portion and the indicia of the adjustment handle form no part of the claimed design. The broken line showing of a portion of the stool illustrates the environment of the claimed design and forms no part thereof.