[Doc] fix third-party model example (#9771)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

docs/source/models/adding_model.rst

@@ -133,6 +133,8 @@ If you are running api server with :code:`vllm serve <args>`, you can wrap the e
     from vllm import ModelRegistry
     from your_code import YourModelForCausalLM
     ModelRegistry.register_model("YourModelForCausalLM", YourModelForCausalLM)
+
+    if __name__ == '__main__':
         import runpy
         runpy.run_module('vllm.entrypoints.openai.api_server', run_name='__main__')