groupingByConcurrent javadoc shows an example use that does not require a map factory

[Arul Dhesiaseelan]

Looks like this example in Collectors javadoc needs to be corrected for
groupingByConcurrent that does not take a map factory?

          ConcurrentMap<City, Set<String>> namesByCity
              =
people.stream().collect(groupingByConcurrent(Person::getCity, TreeMap::new,

 mapping(Person::getLastName, toSet())));

map factory should not be included in this example, as it uses the default
factory.

- Arul