Using the correct auth headers for FTXUS
Using the correct auth headers for FTXUS

FTX.us exchange uses the `FTXUS` prefix in the auth header instead of `FTX`
```
  exchange = ccxt.ftx ({
       'apiKey': 'my secret',
       'secret': 'my key',
       'hostname': 'ftx.us',
       'name': 'FTXUS',
   })
```
ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt,ccxt/ccxt