add update to venv

Signed-off-by: Asra Ali <asraa@google.com>

docs/TESTING.md

@@ -2,7 +2,7 @@
 
 The [Python interoperability tests](client/python_interop/) require Python 3
 (available as `python` on the `$PATH`) and the [`python-tuf`
-package](https://github.com/theupdateframework/python-tuf) installed. To use the correct vesions of the packages, it is recommended to use a virtual environment and install the dependencies via:
+package](https://github.com/theupdateframework/python-tuf) installed. To use the correct vesions of the packages, it is recommended to use a [virtual environment](https://docs.python.org/3/library/venv.html#module-venv) and install the dependencies via:
 
 ```
 python -m pip install --upgrade -r requirements-test.txt