Currently array brackets are required for the --ldapUserToDN config parameter. Omitting bracket throws an exception as the code tries to create a BSONArray. Support:

--ldapUserToDNMapping  '{match:"(.+)",substitution:"cn={0},ou=Users,dc=mongodb,dc=com"}'

in addition to:

--ldapUserToDNMapping  '[{match:"(.+)",substitution:"cn={0},ou=Users,dc=mongodb,dc=com"}]'