updated username format (#628)

Author: kyuan-mongodb

source/includes/steps-launch-charts-saas.yaml

@@ -111,7 +111,7 @@ content: |
       :atlas:`MongoDB users </security-add-mongodb-users>` in your 
       |service| project to access your cluster data. 
       The names of these users are in the format:
-      ``mongodb-stitch-charts-<PROJECT-NAME>-<ID>_<CLUSTER-NAME>``
+      ``charts+<CLUSTER-ID>@mongodb``
 
    .. tabs::