The ornamental design for an adjuster for an armrest, as shown and described.
FIG. 1 is a top side perspective view of an adjuster for an armrest showing our new design;
FIG. 2 is a bottom side perspective view of the adjuster for an armrest as shown in FIG. 1;
FIG. 3 is a front elevational view of the adjuster for an armrest as shown in FIG. 1;
FIG. 4 is a rear elevational view of the adjuster for an armrest as shown in FIG. 1;
FIG. 5 is a left side elevational view of the adjuster for an armrest as shown in FIG. 1;
FIG. 6 is a right side elevational view of the adjuster for an armrest as shown in FIG. 1;
FIG. 7 is a top plan view of the adjuster for an armrest as shown in FIG 1; and,
FIG. 8 is a bottom plan view of the adjuster for an armrest as shown in FIG 1.
The broken lines shown in the drawings are for the purpose of illustrating portions of the adjuster for an armrest which form no part of the claimed design.