The ornamental design for a string for a racket, as shown and described.
FIG. 1 is a perspective view of a string for racket, according to the invention.
FIG. 2 is a right side view of the string for a racket of FIG. 1; and,
FIG. 3 is an end view of the string for a racket of FIG. 1.
The double dotted lines illustrate environment and form no part of the claimed design. The dashed lines form the boundary of the claimed design on the article and form no part of the claimed design. The left side view is a mirror image of the right side view.