Peter Benchley, Author of Jaws, was in town this week. He came not to oversee the movie production of his book but to play a minor role in it. Richard D. Zantuck, one of the producers, called the tall, handsome former newscaster last Monday and asked him if he would like to make his debut as the newscaster on the beach in amity. Mr. Benchley — who fancies himself more like Hooper, the shark expert — was on the set two days later, collecting wages.