Update formatting of Package metadata

Update the capitalization of the Package stage/group in the metadata

doc/architecture/blueprints/container_registry_metadata_database/index.md

 ---
-stage: package
-group: package
+stage: Package
+group: Package
 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/#designated-technical-writers
 comments: false
 description: 'Container Registry metadata database'