Update Google Gemini model name in README

[Mchristos]

Using gemini/gemini-pro yields error:

litellm.exceptions.NotFoundError: litellm.NotFoundError: Vertex_ai_betaException - b'{\n  "error": {\n    "code": 404,\n    "message": "models/gemini-pro is not found for API version v1beta, or is not supported for generateContent. Call ListModels to see the list of available models and their supported methods.",\n    "status": "NOT_FOUND"\n  }\n}\n'

gemini-pro-latest doesn't seem to be mapped by litellm either

Exception: This model isn't mapped yet. model=gemini/gemini-pro-latest, custom_llm_provider=gemini. Add it here - https://github.com/BerriAI/litellm/blob/main/model_prices_and_context_window.json.

So best bet is gemini-2.5-pro or gemini-2.5.flash

[gitnotebooks]

Review these changes at https://app.gitnotebooks.com/AnswerDotAI/shell_sage/pull/62