doc: Use "console" for the highlight language of console output

Using "console" language is better for the console example (including
command-line prompt)

doc/man/sphinx-autogen.rst

@@ -75,7 +75,7 @@ and assuming ``docs/index.rst`` contained the following:
 
 If you run the following:
 
-.. code-block:: bash
+.. code-block:: console
 
     $ PYTHONPATH=. sphinx-autogen docs/index.rst