docs: Advocate for 'python -m pip install -e .' in CONTRIBUTING

CONTRIBUTING.md

@@ -109,7 +109,7 @@ Next, configure your Python environment:
 7. Create a Python virtual environment using `python -m venv .venv`
 8. Activate it using `source .venv/bin/activate`
 9. Upgrade the development dependencies using `python -m pip install -U pip setuptools wheel flit tox`
-10. Install the code in development mode using `flit install --symlink`
+10. Install the code in development mode using `python -m pip install -e .`
     (this means that the installed code will change as soon as you change it in the
     download location).