Update link to data classification policy

What does this MR do?

Fixing Broken Link - haiku by @greg

Hyperlink is broke.

Docs need merge request to fix.

Fixes broken link.

• https://about.gitlab.com/handbook/engineering/security/data-classification-policy.html 😩
• https://about.gitlab.com/handbook/engineering/security/data-classification-standard.html 😀

Sibling MR: gitlab-com/www-gitlab-com!62119 (merged)

@JamesRLopes @axil @sselhorn @JohnathanHunt @ralfaro @jburrows001