The ornamental design for an umbrella, as shown and described.
FIG. 1 is a front elevation view of my new design for an umbrella in a closed position; the back elevation view being a mirror image;
FIG. 2 is a right-side elevation view thereof in a closed position; the left-side elevation view being a mirror image;
FIG. 3 is a front elevation view thereof in an open position; the back elevation view being a mirror image;
FIG. 4 is a right-side elevation view thereof in an open position; the left-side elevation view being a mirror image;
FIG. 5 is an enlarged back elevation view of the umbrella's handle; and,
FIG. 6 is an enlarged left-side elevation view of the umbrella's handle.
The broken lines in the drawings are included for the purpose of illustrating environmental subject matter and form no part of the claimed design. The portions of the umbrella not shown form no part of the claimed design.