crypto/tls: fix link to more info about channel bindings

crypto/tls: fix link to more info about channel bindings

Link in the description of TLSUnique field of ConnectionState struct
leads to an article that is no longer available, so this commit
replaces it with link to a copy of the very same article on another
site.

Fixes #18842.

Change-Id: I8f8d298c4774dc0fbbad5042db0684bb3220aee8
Reviewed-on: https://go-review.googlesource.com/36052
Reviewed-by: Filippo Valsorda <hi@filippo.io>
Reviewed-by: Adam Langley <agl@golang.org>