[webauthn] Clarify names for crypto algs, e.g. RS256

rlin1 has just created a new issue for 
https://github.com/w3c/webauthn:

== Clarify names for crypto algs, e.g. RS256 ==
JSON Web Key defines RS256 which is equivalent to the Web Crypto 
defined "RSASSA-PKCS1-v1_5" (see http://www.w3c.org/TR/WebCryptoAPI/)

In Section 3.8.3 in web authentication spec, we reference the 
WebCrypto terminology, but in example two (and other places) we use 
RS256 - the JWK term.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/123 using your GitHub account