Update metadata entry

Update metadata for spelling and assignment.

doc/subscriptions/gitlab_com/index.md

 ---
-stage: fulfillment
-group: fulfillment
+stage: Fulfillment
+group: Purchase
 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: index, reference
 ---