The ornamental design for a lampshade, as shown and described.
FIG. 1 is a perspective view of a lampshade showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a top & bottom plan view thereof; and,
FIG. 6 is an illustrative view showing a use-state of a lampshade of my design.
The broken lines shown are included for the purpose of illustrating environmental structure and form no part of the claimed design. The different shading used in the drawings demonstrates a contrast in appearance.