Remove `config.protocol` from the README.
Remove `config.protocol` from the README.

Fixes #385.

This was added in error, as part of #344, in
2febadcd028d51e73d8187745c54dabefd2597d2.

The `PDFKit` constructor _does_ support a `protocol` option, but
`PDFKit::Configuration` doesn't.

ContinuityControl/pdfkit,pdfkit/pdfkit