Update docs/user-guide/openai-api/langchain.md: set the model name to NA in the API server

[PeterD1524]

The command to start the API server should set the model name of the chat model to NA (with the option -m, --model-name <MODEL_NAME>) because the chatbot web app assumes that the model is NA.
Otherwise, you will get some output like this saying that the model NA does not exist in the chat graphs:

[2024-07-02 06:58:10.695] [wasi_logging_stdout] [error] llama_core: llama_core::chat in llama-core/src/chat.rs:1033: The model NA does not exist in the chat graphs.
[2024-07-02 06:58:10.695] [wasi_logging_stdout] [error] chat_completions: llama_api_server::backend::ggml in llama-api-server/src/backend/ggml.rs:428: Failed chat completions in non-stream mode. Reason: The model NA does not exist in the chat graphs.
[2024-07-02 06:58:10.695] [wasi_logging_stdout] [error] response: llama_api_server::error in llama-api-server/src/error.rs:25: 500 Internal Server Error: Failed chat completions in non-stream mode. Reason: The model NA does not exist in the chat graphs.
[2024-07-02 06:58:10.695] [wasi_logging_stdout] [info] chat_completions_handler: llama_api_server::backend::ggml in llama-api-server/src/backend/ggml.rs:317: Send the chat completion response.
[2024-07-02 06:58:10.695] [wasi_logging_stdout] [error] response: llama_api_server in llama-api-server/src/main.rs:522: version: HTTP/1.1, body_size: 128, status: 500, is_informational: false, is_success: false, is_redirection: false, is_client_error: false, is_server_error: true

https://discord.com/channels/846973236280950824/938097393369489448/1257592298385051718

NA is the default model name in LlamaEdgeChatService:
langchain-ai/langchain/libs/community/langchain_community/chat_models/llama_edge.py#L72-L83

A better solution might be to have the chatbot web app use the /v1/models endpoint to get the list of available models and display a dropdown to select the model to use, but I don't know how to do this with LangChain.

[apepkuss]

Thanks for this PR. We plan to update LlamaEdgeService in the near future as LlamaEdge itself gets a lot of improvements in recent releases. We'll come back to review this PR after finishing the necessary updates on LlamaEdgeService. So, please be patient for the review. Thanks a lot!