Fix a link and corresponding text on the "Financial Planning & Analysis" page.
This updates the "Engineering Management Handbook Page" link and corresponding text at https://about.gitlab.com/handbook/finance/financial-planning-and-analysis/#retrospectives.
- The link text has been changed from "Engineering Management Handbook Page" to "Team Retrospectives" to better describe the linked content.
- The surrounding text has been adjusted for clarity.
- The link URL has been made root-relative.