I claim the ornamental design for a lighting filament, as shown and described.
FIG. 1 is a perspective view of a lighting filament showing the new design;
FIG. 2 is a front elevation view of the lighting filament of FIG. 1;
FIG. 3 is a rear elevation view of the lighting filament of FIG. 1;
FIG. 4 is a left side elevation view of the lighting filament of FIG. 1;
FIG. 5 is a right side elevation view of the lighting filament of FIG. 1;
FIG. 6 is a top plan view of the lighting filament of FIG. 1; and,
FIG. 7 is a bottom plan view of the lighting filament of FIG. 1.
The broken lines immediately adjacent shaded areas represent the bounds of the claimed design, while all other broken lines are directed to environment or portions of the lighting filament that form no part of the claim design. None of the broken lines form any part of the claimed design.