Rename "Sustainability" section to "Cost efficiency" in README

Author: robertpiosik

README.md

@@ -37,7 +37,7 @@ Code Web Chat (CWC) is a free and open-source, privacy-first **AI coding toolkit
 
 <p align="center"><img src="https://github.com/robertpiosik/CodeWebChat/raw/HEAD/media/screenshot-3.png"></p>
 
-## Sustainability 🌱
+## Cost efficiency
 
 First-class support for [prompt caching](https://developers.openai.com/api/docs/guides/prompt-caching/) by ordering the selected context files by update recency. Make your chatbot quotas last longer or lower token costs by up to 90% when calling APIs. Forget about slow prompt processing with local inference.