Clarify wording on authenticator selection

[yaronf]

8.2 Authenticator Selection: The text is unclear. Instead of "If the client supports the Authenticator Selection Extension, it MUST use the first available authenticator whose AAGUID is present in the AuthenticatorSelectionList" I would suggest: "If the client supports the Authenticator Selection Extension, it MUST use the first authenticator from the AuthenticatorSelectionList that is available in the system".

Rationale: the list obviously has an order, whereas the "first available authenticator" is not well defined.

[vijaybh]

Duplicate of #95

[yaronf]

Agree, Closing.