Fixed API return type in documentation (#28984)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

api/v4/source/definitions.yaml

@@ -3059,6 +3059,19 @@ components:
           format: int64
         has_syncables:
           type: boolean
+    GroupMember:
+      type: object
+      properties:
+        group_id:
+          type: string
+        user_id:
+          type: string
+        create_at:
+          type: integer
+          format: int64
+        delete_at:
+          type: integer
+          format: int64
     GroupSyncableTeam:
       type: object
       properties: