update after_script for appplatform's Python SDK (#10563)

Author: yuwzho

specification/appplatform/resource-manager/readme.md

@@ -79,6 +79,8 @@ This is not used by Autorest itself.
 ``` yaml $(swagger-to-sdk)
 swagger-to-sdk:
   - repo: azure-sdk-for-python
+    after_scripts:
+      - python ./scripts/multiapi_init_gen.py azure-mgmt-appplatform
   - repo: azure-sdk-for-java
   - repo: azure-sdk-for-net
   - repo: azure-sdk-for-node