The ornamental design for a bowstring, as shown and described.
FIG. 1 is a front view of my ornamental design for a bowstring showing two ends having a contrast, the back view of the bowstring is identical to the front view;
FIG. 2 is a left side view of the bowstring of FIG. 1, the right-side view is identical to the left-side view;
FIG. 3 is a first end-view of the bowstring of FIG. 1; and,
FIG. 4 is a second end-view of the bowstring of FIG. 1.
An ornamental bowstring, wherein a contrast between the two ends of the bowstring is being claimed. The design is shown broken apart along its length in two locations to indicate that no specific length is claimed.