Fix tune download command in PPO config

recipes/configs/mistral/7B_full_ppo_low_memory.yaml

@@ -7,7 +7,7 @@
 #
 # This config assumes that you've run the following command before launching
 # this run:
-#   tune download weqweasdas/RM-Mistral-7B --output-dir /tmp/RM-Mistral-7B/ --ignore-patterns=""
+#   tune download weqweasdas/RM-Mistral-7B --output-dir /tmp/RM-Mistral-7B/ --ignore-patterns None
 #   tune download mistralai/Mistral-7B-Instruct-v0.2 --output-dir /tmp/Mistral-7B-Instruct-v0.2/ --hf-token HF_TOKEN
 #
 # You'll also need to ensure that {output_dir} exists beforehand, as checkpoints for policy and value models are saved in sub-folders.