The ornamental design for an apron, as shown and described.
FIG. 1 is a front elevation view of an apron showing the new design;
FIG. 2 is a rear elevation view of the apron in FIG. 1;
FIG. 3 is a left side elevation view of the apron in FIG. 1; and,
FIG. 4 is a right side elevation view of the apron in FIG. 1.
The partial human figure shown in broken lines is the environmental subject matter included to illustrate how the design may appear when worn and forms no part of the claimed design.