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

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

docs/source/models/adding_model.rst

@@ -133,7 +133,9 @@ 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)
-    import runpy
-    runpy.run_module('vllm.entrypoints.openai.api_server', run_name='__main__')
+
+    if __name__ == '__main__':
+        import runpy
+        runpy.run_module('vllm.entrypoints.openai.api_server', run_name='__main__')
 
 Save the above code in a file and run it with :code:`python your_file.py <args>`.