Remove old tox env (linting) from contributing doc (#337)

Author: attomos

docs/contributing.rst

@@ -15,7 +15,7 @@ Tests are run with ``tox``, you can run the baseline environments before submitt
 
 .. code-block:: console
 
-    $ tox -e py38,linting
+    $ tox -e py38
 
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 `pre-commit <https://pre-commit.com>`_.