An ornamentaldesign for a spa, as shown and described.
FIG. 1 is a perspective view of the spa of the instant design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is an end view thereof as seen from the right hand side of FIG. 1;
FIG. 5 is a side elevational view thereof as seen from the bottom of FIG. 2 with the other side view being a mirror image thereof; and,
FIG. 6 is an end view thereof as seen from the left hand side of FIG. 2.
The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design.