Update README.md

Signed-off-by: Darren Shepherd <darren@acorn.io>

Author: ibuildthecloud

README.md

@@ -93,7 +93,7 @@ export OPENAI_AZURE_DEPLOYMENT="<your-deployment-name>"
 #### Windows
 
 ```powershell
-$env:OPENAI = 'your-api-key'
+$env:OPENAI_API_KEY = 'your-api-key'
 ```
 
 ### 3. Run Hello World