chore(docs): update server run command

Author: MickaelCa

CONTRIBUTING.md

@@ -65,7 +65,7 @@ If you ever get stuck, reach out on [Discord](https://discord.com/invite/zerRaGK
 9. **Run the local server** to sanity-check:
 
     ```bash
-    python run_server.py
+    python -m server
     ```
 
     Open [http://localhost:8000](http://localhost:8000) to confirm everything works.