Fix unsafe `update` example (#4817)
Fix unsafe `update` example (#4817)

It's not generally safe to mutate the result of `readQuery` directly as in some
circumstances the object you are mutating is shared with an object in the cache,
so you can get some weird bugs from doing this.
apollographql/apollo-client,apollostack/apollo-client,apollostack/apollo-client,apollostack/apollo-client,apollographql/apollo-client