update acp/authentication language var

the language var ACP_AUTH_SETTINGS_EXPLAIN in language/en/acp/board.php
should be updated from:
phpBB supports authentication plug-ins, or modules. These allow you determine how users are authenticated when they log into the board. By default three plug-ins are provided; DB, LDAP and Apache. Not all methods require additional information so only fill out fields if they are relevant to the selected method.
to:
phpBB supports authentication plug-ins, or modules. These allow you determine how users are authenticated when they log into the board. By default four plug-ins are provided; DB, LDAP, Apache, and Oauth. Not all methods require additional information so only fill out fields if they are relevant to the selected method.