Error at self-signed-ssl-server code sample on README

There's an error at https://github.com/joltup/rn-fetch-blob#self-signed-ssl-server

```javascript
RNFetchBlob.config({
  trusty : true
})
.then('GET', 'https://mysite.com') // wrong line
.then((resp) => {
  // ...
})
```

should be

```javascript
RNFetchBlob.config({
  trusty : true
})
.fetch('GET', 'https://mysite.com')
.then((resp) => {
  // ...
})
```
