Fix the usage of constructing IOST instance in README

[nujabes403]

According to index.js, module.exports are defined like below:

module.exports = {
    IOST: IOST,
    RPC: RPC,
    HTTPProvider: HTTPProvider,
    KeyPair: KeyPair,
    Tx : Tx,
    Algorithm: Algorithm,
    Account: Account,
    TxHandler: TxHandler,
}

To instantiate IOST, you should do it with a statement new IOST.IOST({...}) instead new IOST({...}).

Currently, instantiating IOST is wrongly described in README.md usage.

It should be fixed.

[nujabes403]

Please take a look. @sswsdsn