I claim the ornamental design for a nail brush, as shown and described.
FIG. 1 is a perspective view of a nail brush showing my new design;
FIG. 2 is a front view thereof, the rear view being a mirror image of the front view;
FIG. 3 is a left side view thereof, the right side view being a mirror image of the left side view;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is another perspective view thereof;
FIG. 7 is a partially enlarged view showing the area 7 in FIG. 1; and,
FIG. 8 is a partially enlarged view showing the area 8 in FIG. 3.
The broken lines showing the brush tip in the figures depict environment and forms no part of the claimed design. All other broken lines depict portions of the nail brush that form no part of the claimed design. The circles shown in dot-dash in the Figures show enlarged views and form no part of the claimed design.