XMLSigner's sign method's `key` parameter represents private key

[wesley6j]

I think these should be private keys?

:param key:
            Key to be used for signing. When signing with a certificate or RSA/DSA/ECDSA key, this can be a string
            containing a PEM-formatted key, or a :py:class:`cryptography.hazmat.primitives.interfaces.RSAPublicKey`,
            :py:class:`cryptography.hazmat.primitives.interfaces.DSAPublicKey`, or
            :py:class:`cryptography.hazmat.primitives.interfaces.EllipticCurvePublicKey` object. When signing with a
            HMAC, this should be a string containing the shared secret.
:type key:
            string, :py:class:`cryptography.hazmat.primitives.interfaces.RSAPublicKey`,
            :py:class:`cryptography.hazmat.primitives.interfaces.DSAPublicKey`, or
            :py:class:`cryptography.hazmat.primitives.interfaces.EllipticCurvePublicKey` object

https://github.com/XML-Security/signxml/blob/master/signxml/__init__.py#L294

[kislyuk]

I think you're right, that's a typo in the docstring.