[black] Add --diff to checks

Author: stephane-caron

tox.ini

@@ -37,7 +37,7 @@ deps =
     robomeshcat >= 1.0.4
     yourdfpy >=0.0.52
 commands =
-    black --check robot_descriptions
+    black --check --diff robot_descriptions
     flake8 robot_descriptions
     pylint robot_descriptions --exit-zero --rcfile={toxinidir}/tox.ini
     mypy robot_descriptions --ignore-missing-imports