Update titles to Merge request approval policies

What does this MR do and why?

This MR updates the titles in the navigation to be in line with the headings of the documents that were changed in:

• Update documentation for Scan result policies (gitlab!143473 - merged)
• Update doc instances of scan result policies (gitlab!143583 - merged)

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.