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/36052Reviewed-by: Filippo Valsorda <hi@filippo.io>
Reviewed-by: Adam Langley <agl@golang.org>