The ornamental design for a collapsible shelf, as shown and described.
Description:
FIG. 1 is a top perspective view of a collapsible shelf showing my new design, with the broken line showing of side supports for illustrative purposes only and forming no part of the claimed design;
FIG. 2 is an enlarged front elevational view thereof;
FIG. 3 is a right side elevational view of FIG. 2;
