Update ownership of Invalidate Markdown Cache docs

doc/administration/invalidate_markdown_cache.md

 ---
-stage: Create
-group: Source Code
+stage: Plan
+group: Project Management
 info: "To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments"
 type: reference
 ---