fix missing class object instantiation in custom_llm_server provider documentation's quick start (#5578)

Co-authored-by: Pradyumna Singh Rathore <pradyumna.singhrathore@halliburton.com>

docs/my-website/docs/providers/custom_llm_server.md

@@ -23,6 +23,8 @@ class MyCustomLLM(CustomLLM):
             mock_response="Hi!",
         )  # type: ignore
 
+my_custom_llm = MyCustomLLM()
+
 litellm.custom_provider_map = [ # 👈 KEY STEP - REGISTER HANDLER
         {"provider": "my-custom-llm", "custom_handler": my_custom_llm}
     ]