The ornamental design for a spa, as shown and described.
FIG. 1 is a top and right side perspective view of a spa showing my new design;
FIG. 2 is a top perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left-side interior sectional view thereof, taken along line 4—4 of FIG. 3;
FIG. 5 is a right-side interior sectional view thereof, taken along line 5—5 of FIG. 3;
FIG. 6 is a rear interior sectional view thereof, taken along line 6—6 of FIG. 3; and,
FIG. 7 is a front interior sectional view thereof, taken along line 7—7 of FIG. 3.
The broken lines that form the exterior of the claimed design represent the boundaries of the claimed design while all other broken lines are directed to unclaimed subject matter. None of the broken lines form part of the claimed design.