update instructions for UML image generation

Author: drbenvincent

CONTRIBUTING.md

@@ -172,7 +172,7 @@ Docs are built in `docs/_build`, but these docs are _not_ committed to the GitHu
 UML diagrams can be created with the command below. If you have not already done so, you may need to `pip install 'causalpy[lint]'` in order to install `pyreverse`.
 
 ```bash
-pyreverse -o png causalpy --output-directory img
+pyreverse -o png causalpy --output-directory docs/source/_static
 ```
 
 Classes