Doc: Pacemaker Administration: Fix CIB_user default

For remote connections, the default value for CIB_user is hacluster, not
$USER. See cib_new().

For other purposes (for example, ACLs), the default is either NULL or
the current EUID, depending on the context. That can be addressed later.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>