Typo in readme: expense receipt token

Just a small typo in the readme, I found the real value by looking through the code base.

This is in the readme:

```
from mindee import Client

mindee_client = Client(
    expense_receipts_token="your_expense_receipts_api_token_here",
```

but the named parameter should be `expense_receipt_token`.

`receipts` vs `receipt`