fixup! add concrete example for single test

Author: EricCousineau-TRI

.github/CONTRIBUTING.md

@@ -205,6 +205,12 @@ This is useful if you do not test against all required dependency versions.
 python -m pytest pytorch_lightning tests pl_examples -v --flake8
 ```
 
+You can also run a single test as follows:
+
+```bash
+python -m pytest -v ./tests/trainer/test_trainer_cli.py::test_default_args
+```
+
 ### Pull Request
 
 We welcome any useful contribution! For your convenience here's a recommended workflow: