The ornamental design for a spa, as shown and described.
FIG. 1 is a perspective view of a spa showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right-side elevation view thereof; and,
FIG. 6 is a top plan view thereof.
The bottom views form no part of the design and are omitted.
The broken lines immediately adjacent the shaded areas represent the bounds of the claim, while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.