Update CONTRIBUTING.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: GiannisGeo <giannisgeorgiadiis@gmail.com>

Author: Giannis-dot

CONTRIBUTING.md

@@ -184,7 +184,7 @@ python -m pip install -U pip
 python -m pip install pytest
 
 # run a single test file
-python -m pytest -q tests/test_subscription_handle.py
+python -m pytest -q tests/unit/test_subscription_handle.py
 ```
 
 If you want to run a subset of tests, use `-k` with a keyword expression or provide a directory/file path. This helps avoid heavy `conftest.py` imports during quick iteration.