fix docs for Swivel.signOrder

[alexanderwende]

    /**
     * Sign an order using EIP-712.
     *
     * @remarks
     * https://eips.ethereum.org/EIPS/eip-712
     *
     * @param o - order to sign
     * @param i - chain-id for the deployed smart contract
     * @param c - address of a deployed verifying contract
     */
    async signOrder (o: Order): Promise<string> {

        if (!this.chainId || !this.verifyingContract) throw CHAIN_ID_AND_VERIFYING_CONTRACT_REQUIRED;

        return await this.vendor.signOrder(o, this.chainId, this.verifyingContract);
    }

@param i and @param c are copy-paste leftovers from vendor method