The ornamental design for an umbrella, as shown and described.
FIG. 1 is a front perspective view of an umbrella emboding my novel design;
FIG. 2 is a right side elevational view thereof, the left side elevational view thereof being a mirror image;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational plan view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The canopy or covering for the umbrella is formed of a transparent material, so that the appearance of the umbrella is dictated not only by a view to one side of the canopy but the total view also through to the other side of the canopy.