Update docs/config/version_provider.md

Co-authored-by: Wei Lee <weilee.rx@gmail.com>

Author: bearomorphism

docs/config/version_provider.md

@@ -235,7 +235,7 @@ dependencies = ["commitizen"]
 my-provider = "my_provider:MyProvider"
 ```
 
-**Using `setup.py`:**
+**Using `setup.py` (for legacy setup):**
 
 ```python title="setup.py"
 from setuptools import setup