fix readme env vars (All-Hands-AI#199)

README.md

@@ -41,13 +41,12 @@ You'll see OpenDevin running at localhost:3001
 We use LiteLLM, so you can run OpenDevin with any foundation model, including OpenAI, Claude, and Gemini.
 LiteLLM has a [full list of providers](https://docs.litellm.ai/docs/providers).
 
-To change the model, set the `MODEL_NAME` environment variable, as well as the appropriate
-API key based on LiteLLM's [provider documentation](https://docs.litellm.ai/docs/providers).
+To change the model, set the `LLM_MODEL` and `LLM_API_KEY` environment variables.
 
 For example, to run Claude:
 ```bash
-export ANTHROPIC_API_KEY="your-api-key"
-export MODEL_NAME="claude-3-opus-20240229"
+export LLM_API_KEY="your-api-key"
+export LLM_MODEL="claude-3-opus-20240229"
 ```
 
 ## 🤔 What is [Devin](https://www.cognition-labs.com/introducing-devin)?