Remove trailing `]` from about page security link

[jeff-horton-ho-sas]

Content change

• [X] Please review the accessibility checks for content changes.

I can confirm:

• [X] Content does not include any code or configuration changes (excluding frontmatter information)
• [X] Content meets the content standards e.g. Writing a principle and Writing a standard
• [X] Content is suitable to open source, i.e.:
  • Content does not relate to unreleased gov policy
  • Content does not refer to anti-fraud mechanisms
  • Content does not include sensitive business logic
• ~[X] Last updated date for content is correct~ N/A - change to a site page, not a pattern, standard, or principle.