update [#getAssertion](https://w3c.github.io/webauthn/#getAssertion) section a la PR #498. Doing this will complete webauthn alignment with credman PR https://github.com/w3c/webappsec-credential-management/pull/100, and fix #254 Note: https://github.com/w3c/webappsec-credential-management/pull/100 needs completion & merging
update #getAssertion section a la PR #498. Doing this will complete webauthn alignment with credman PR w3c/webappsec-credential-management#100, and fix #254
Note: w3c/webappsec-credential-management#100 needs completion & merging