your update statement is not picking dvd_id from parent table rebtal, but evaluating like where dvd_id = dvd_id which will always be TRUE. Just ad :OLD qualifier and you should be good, considering this is same column name (dvd_id) in rental table.
CREATE OR REPLACE TRIGGER RENTAL_RETURNS
AFTER UPDATE OF DATE_RETURN ON RENTAL
FOR EACH ROW
BEGIN
IF :OLD.DATE.RETURN IS NULL AND :NEW.DATE_RETURN IS NOT NULL THEN
UPDATE DVD SET RENT_AVAIL = 'Y' WHERE DVD.DVD_ID = :OLD.DVD_ID;
END IF;
END;