Fix template readme (Azure#28133)

* readme template

* fix readme

Author: msyyc

tools/azure-sdk-tools/packaging_tools/templates/packaging_files/README.md

@@ -33,10 +33,11 @@ By default, [Azure Active Directory](https://aka.ms/awps/aad) token authenticati
 In addition, Azure subscription ID can be configured via environment variable `AZURE_SUBSCRIPTION_ID`.
 {% endif %}
 With above configuration, client can be authenticated by following code:
+{%- set sub_namespace = "." + sub_namespace if sub_namespace else "" %}
 
 ```python
 from azure.identity import DefaultAzureCredential
-from {{package_name | replace("-", ".")}} import {{ title }}
+from {{package_name | replace("-", ".")}}{{sub_namespace}} import {{ title }}
 {% if not no_sub -%}
 import os