Align provisioningURI with docs

The [how to use](https://github.com/Spomky-Labs/otphp/blob/master/doc/Use.md) doc suggests the following:

```
$totp->provisioningURI("alice@google.com"); // => 'otpauth://totp/alice@google.com?secret=JBSWY3DPEHPK3PXP'
```

However, this actually throws a `No label defined.` exception. This should fix that.