Possible 1.1 (and 1.0) errata

[Scott Cantor <cantor.2@osu.edu>]

It's been pointed out during review of the latest Liberty documents that the
part in SAML about identifier uniqueness is overstated based on the intent.
If the point is to use a SHA1 hash, then the actual collision probability in
the spec language should be <= 2^-80 instead of < 2^-160

Liberty had the same language and it was copied from SAML, so I figured I'd
mention it.

-- Scott