The ornamental design for a retentionspoon, as shown and described.
FIG. 1 is a top plan view of a retention spoon showing my new design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a left perspective view thereof; and,
FIG. 4 is a right side elevational view thereof, the left side being a mirror image.
The broken line showing of a spoon handle in FIGS. 1-4 is included for the purpose of illustrating environmental structure and forms no part of the claimed design.