Update method name in wmill README sample code (#2679)

In the README's example code for the wmill Python client, the method name `start_execution` has been changed to `run_script_async`. This change is in line with the recent updates we made to the client's public API. The naming adjustment adds more clarity to the method's functionality, and ensures that the sample code in the README is accurate and stays updated with the latest changes in the wmill Python client's API.

python-client/wmill/README.md

@@ -17,7 +17,7 @@ def main():
     print(client.version)
     print(client.get("u/user/resource_path"))
 
-    job_id = client.start_execution(path="path/to/script")
+    job_id = client.run_script_async(path="path/to/script")
     print(job_id)
 
     return client.run_script(path="path/to/script", args={"arg1": "value1"})