The ornamental design for an heart-shaped umbrella, as shown and described.
FIG. 1 is a perspective of a heart-shaped umbrella showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof; and,
FIG. 6 is a right side elevation view thereof.
The broken lines in the drawings are included for the purpose of illustrating portions of the heart-shaped canopy that form no part of the claimed design.