An ornamentaldesign for a string instrument, as shown and described.
FIG. 1 is a front perspective view of the string instrument.
FIG. 2 is a front elevational view of the string instrument.
FIG. 3 is a rear elevational view of the string instrument.
FIG. 4 is a left side view of the string instrument.
FIG. 5 is a right side view of the string instrument.
FIG. 6 is a top view of the string instrument; and,
FIG. 7 is a bottom view of the string instrument.
The shading is provided to illustrate contour only and is not an indication of texture, gloss, etc. The broken lines illustrate the portions of the string instrument that form no part of the claimed design.