Fixed API return type in documentation (#28984)

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

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: