[ticket/11844] update acp/authentication language var

the language var ACP_AUTH_SETTINGS_EXPLAIN in language/en/acp/board.php

PHPBB3-11844
Merge remote-tracking branch 'Prosk8er/ticket/11844' into develop

# By Prosk8er
# Via Prosk8er
* Prosk8er/ticket/11844:
  [ticket/11844] update acp/authentication language var