The ornamental design for a fingernailmargintool, as shown and described.
FIG. 1 is a perspective view of a fingernail margin tool showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a right-side view thereof, with the fingernail margin tool turned upside down;
FIG. 6 is a top view thereof; and,
FIG. 7 is a left-side view thereof.
The broken line illustration of numbers shown in FIGS. 1 and 6 represents unclaimed portions of the fingernail margin tool, and forms no part of the claimed design.