_content/security: update index.md
_content/security: update index.md

Change-Id: I00be3d71f1d832d3077c83bbf2f9ef301295bc55
Reviewed-on: https://go-review.googlesource.com/c/website/+/427478
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