Fixed typo in the exception message

[addison74]

In file /app/code/core/Mage/Payment/Helper/Data.php there is a typo. Unkown => Unknown.

    public function getMethodInstance($code)
    {
        $key = self::XML_PATH_PAYMENT_METHODS . '/' . $code . '/model';
        $class = Mage::getStoreConfig($key);
        if (is_null($class)) {
            Mage::logException(new Exception(sprintf('Unkown payment method with code "%s"', $code)));
            return false;
        }
        return Mage::getModel($class);
    }