The ornamental design for a beard trimmer, as shown and described.
FIG. 1 is a perspective view showing a beard trimmer design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The portions of the beard trimmer shown in broken lines are included for the purpose of illustration unclaimed portions of the beard trimmer and form no part of the claimed design.