Fix request_ts_micro typo

[Fayne]

Dear package owner，

I think there is a typo here. Could you please review my commit and merge it? Thanks a lot.

[Krinkle]

For the benefit of seach and discovery for others - this typo causes a SQL error on the /xhgui/custom/help route, with an error message like this one:

Slim Application Error:

Type: PDOException
Code: 42S22
Message:
  SQLSTATE[42S22]: Column not found: 1054
  Unknown column 'request_ts_micro,' in 'field list'
File: /srv/xhgui/src/Db/PdoRepository.php
Line: 58

Trace:
#0 /srv/xhgui/src/Db/PdoRepository.php(58): PDO->query()
#1 /srv/xhgui/src/Searcher/PdoSearcher.php(26): XHGui\\Db\\PdoRepository->getLatest()