The ornamental design for a cushion for stretching a person's spine, as shown and described.
FIG. 1 is a front perspective view of the cushion for stretching a person's spine, showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a left side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines in the form of a bar in FIGS. 1-6 depict environmental subject matter that forms no part of the claimed design. All other broken lines in the drawings depict portions of the cushion for stretching a person's spine that form no part of the claimed design.