The ornamental design for a heart-shaped stringed instrument, as shown and described.
FIG. 1 is a front perspective view of heart-shaped stringed instrument;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a left side elevational view thereof; and,
FIG. 7 is a back elevational view thereof.
The broken lines immediately adjacent shaded surfaces depict claim boundaries. All other broken lines depict portions of the heart-shaped stringed instrument that form no part of the claim.