SDK assumes networkID as tc if not specified.

Assuming `tc` as default `networkID` is not problem.

But in the first example code of [codechain sdk document](https://api.codechain.io/2.0.1/index.html),
`networkID` is not specified while constructing SDK object.

This might cause confusion because sdk also assumes `networkID` as `tc` in the case below.
```javascript
var sdk = new SDK({ server: "https://corgi-rpc.codechain.io" });
```

To set networkId to `wc`, we should specify it
```javascript
var sdk = new SDK({ server: "https://corgi-rpc.codechain.io"
networkId: "wc"});
```

So I think we should change document or change assuming logic.