_content/security/vuln: use relative link in cna.md
_content/security/vuln: use relative link in cna.md

Change-Id: I4dbd4420a26ca035160c6150ed2499667f4a4aee
Reviewed-on: https://go-review.googlesource.com/c/website/+/427595
Reviewed-by: Brandon Kessler <ca053a7d85137b77c1c549ec7c63bd3f6ad3fa72@google.com>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website