crypto/ecdsa: fix dead reference link
The previous link broke, but it's available on the internet archive.
Fixes #39808
Change-Id: Ic2be74a1f0591600ca1acbe08e1bab8ba1e21abe
GitHub-Last-Rev:
6d6de5d2f451c6d53a1e55b62fb5a1fab0d49f10
GitHub-Pull-Request: golang/go#40165
Reviewed-on: https://go-review.googlesource.com/c/go/+/242103
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>