The ornamental design for a spoon, as shown and described.
FIG. 1 is a perspective view of the spoon of the present design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a front end view thereof; and,
FIG. 6 is a rear end view thereof.
The portions of the Figures shown in phantom lines are not part of the claimed design but are for illustration of environmental aspects only, for example, that the design may illustrate a measuring spoon having a spoon bowl volume indication.