Local test docs: correct tox shell command usage (#49350)

The ansible-test shell command doesn't have a --python option; you
have to specify the tox environment directly to the --tox command

Partially fixes: #49349

docs/docsite/rst/dev_guide/testing_running_locally.rst

@@ -57,7 +57,7 @@ Interactive Shell
 Use the ``ansible-test shell`` command to get an interactive shell in the same environment used to run tests. Examples:
 
 * ``ansible-test shell --docker`` - Open a shell in the default docker container.
-* ``ansible-test shell --tox --python 3.6`` - Open a shell in the Python 3.6 ``tox`` environment.
+* ``ansible-test shell --tox 3.6`` - Open a shell in the Python 3.6 ``tox`` environment.
 
 
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