Adjusted documentation of tableList command (Java)

[clovergaze]

• provided the correct return type
• extended the example

Reason for the change I tried to simply get the list of table names in a database and the current documentation is not helpful in that regard. The specified return type is wrong and the array with table names had to be extracted from the Result<Object>.

Description I corrected the return type and added a useful example that shows how to really get the table names.

Checklist

• [x] I have read and agreed to the RethinkDB Contributor License Agreement

[clovergaze]

This might also be a viable example:

try (Result<Object> result = r.db(DEFAULT_DB_NAME).tableList().run(connection)) {
  result.forEach(list -> {
    ((List<?>) list).forEach(System.out::println);
  });
}