The ornamental design for a vent for a dishwasher, as shown and described.
FIG. 1 is a front perspective view of a vent for a dishwasher showing my new design;
FIG. 2 is an enlarged front perspective view thereof; and,
FIG. 3 is a front view thereof.
The broken line showing of a dishwasher door in FIG. 1 is included for the purpose of showing environment of the vent for a dishwasher and forms no part of the claimed design.