[docs] Snippet did not have a code-block tag above it (#3442)

doc/source/autoscaling.rst

@@ -78,6 +78,8 @@ You can use ``ray exec`` to conveniently run commands on clusters. Note that scr
 
 You can also use ``ray submit`` to execute Python scripts on clusters. This will ``rsync`` the designated file onto the cluster and execute it with the given arguments.
 
+.. code-block:: bash
+
     # Run a Python script in a detached tmux session
     $ ray submit cluster.yaml --tmux --start --stop tune_experiment.py