The ornamental design for an instrument, as shown and described.
FIG. 1 is a perspective view of an instrument showing our new design;
FIG. 2 is a front side elevational view of the instrument of FIG. 1;
FIG. 3 is a rear side elevational view of the instrument of FIG. 1;
FIG. 4 is a right side elevational view of the instrument of FIG. 1;
FIG. 5 is a left side elevational view of the instrument of FIG. 1;
FIG. 6 is a top plan view of the instrument of FIG. 1; and,
FIG. 7 is a bottom plan view of the instrument of FIG. 1.
The broken lines shown in the drawings illustrate portions of the instrument that form no part of the claimed design. The short-long dashed lines represent boundaries of the claimed design and form no part of the claimed design.