[Doc] Clarify sincedb_path default value

[toby-sutor]

In the documentation at https://www.elastic.co/guide/en/logstash/current/plugins-inputs-dead_letter_queue.html#plugins-inputs-dead_letter_queue-sincedb_path

It states: There is no default value for this setting.
But then later it also states: The default will write sincedb files to <path.data>/plugins/inputs/dead_letter_queue.

According to https://github.com/logstash-plugins/logstash-input-dead_letter_queue/blob/master/lib/logstash/inputs/dead_letter_queue.rb#L43 it seems that the second statement is correct. Please adjust the documentation accordingly. Thank you.