The ornamental design for an umbrella, as shown and described.
FIG. 1 is a perspective view of an umbrella;
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 right-side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken line showing of a plurality of ornaments in FIGS. 1-6 is included for the purpose of showing environmental portions of the umbrella and forms no part of the claimed design.