Update index.rst (#339)

```python
>>> decoded_sig = base64url_decode(encoded_sig)
Traceback (most recent call last):
(...)
  File "(...)/python3.11/site-packages/jose/utils.py", line 76, in base64url_decode
    input += b"=" * (4 - rem)
TypeError: can only concatenate str (not "bytes") to str
>>> decoded_sig = base64url_decode(encoded_sig.encode())
>>>
```