Improve DeserializeSeed documentation

[tomasol]

The DeserializeSeed contains an example without a clear way to call it. The last two lines should be surrounded by fn example.., but unless one clicks on the source, those lines will not be shown.
I would like to propose removing the leading '#' characters or perhaps changing the usage to the following:

    #[test]
    fn deserialize_seed() {
        let input = "[1, 2]";
        let mut vec: Vec<u64> = Vec::new();
        let mut deserializer = serde_json::Deserializer::from_str(input);
        ExtendVec(&mut vec).deserialize(&mut deserializer).unwrap();
        assert_eq!(vec![1, 2], vec);
    }

    #[test]
    fn visitor_deserialize_seed() {
        let input = "[[1], [2]]";
        let mut deserializer = serde_json::Deserializer::from_str(input);
        let visitor = FlattenedVecVisitor::<u64>(PhantomData);
        let vec: Vec<u64> = deserializer.deserialize_seq(visitor).unwrap();
        assert_eq!(vec![1, 2], vec);
    }