Update ledger info

        Arguments:
            ledger_id:
                Ledger id assigned by mCASH
            description:
                Description of the Ledger and it's usage

def update_ledger(self, ledger_id, description=None):
        """Update ledger info

        Arguments:
            ledger_id:
                Ledger id assigned by mCASH
            description:
                Description of the Ledger and it's usage
        """
        arguments = {'description': description}
        return self.do_req('PUT',
                           self.merchant_api_base_url + '/ledger/' +
                           ledger_id + '/', arguments)