Update Phalcon_Db_Adapter_Pdo_Postgresql.rst

Code in *fetchOne* change from
$robot = $connection->fecthOne("SELECT * FROM robots");
$robot = $connection->fecthOne("SELECT * FROM robots", Phalcon\Db::FETCH_ASSOC);
to
$robot = $connection->fetchOne("SELECT * FROM robots");
$robot = $connection->fetchOne("SELECT * FROM robots", Phalcon\Db::FETCH_ASSOC);