Fix inconsistent variable reference in README custom base URL section

[Copilot]

The "Using a custom base URL and API key" section referenced CHAT_MODEL in the explanatory text but used MODEL_NAME in the code snippet.

Changes

• Updated line 90 to reference MODEL_NAME consistently with the code example

-Replace `CHAT_MODEL` with your model name and `BASE_URL` with your endpoint URI.
+Replace `MODEL_NAME` with your model name and `BASE_URL` with your endpoint URI.

Original prompt

In the README.md file, under the section called "Using a custom base URL and API key", it makes a reference to CHAT_MODEL, but the code snippet calls it MODEL_NAME. Rename CHAT_MODEL to MODEL_NAME.

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