Update readme.python.md (Azure#22548)

* Update readme.python.md

* Update readme.python.md

---------

Co-authored-by: Yuchao Yan <[email protected]>

Author: 2 people

specification/dns/resource-manager/readme.python.md

@@ -16,6 +16,7 @@ Generate all API versions currently shipped for this package
 
 ```yaml $(python)
 multiapi: true
+default-api-version: "2018-05-01"
 clear-output-folder: true
 batch:
   - tag: package-2018-05
@@ -62,4 +63,4 @@ output-folder: $(python-sdks-folder)/network/azure-mgmt-dns/azure/mgmt/dns/v2016
 ```yaml $(python)
 modelerfour:
   lenient-model-deduplication: true
-```
+```