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:

api/v4/source/groups.yaml

@@ -854,7 +854,9 @@
           content:
             application/json:
               schema:
-                $ref: "#/components/schemas/StatusOK"
+                type: array
+                items:
+                   $ref: "#/components/schemas/GroupMember"
     post:
       tags:
         - groups
@@ -898,7 +900,9 @@
           content:
             application/json:
               schema:
-                $ref: "#/components/schemas/StatusOK"
+                type: array
+                items:
+                   $ref: "#/components/schemas/GroupMember"
   "/api/v4/groups/{group_id}/stats":
     get:
       tags: