add yaml highlight to unit 3 & 5

Author: Yelinz

notebooks/unit3/unit3.ipynb

@@ -310,7 +310,7 @@
         "\n",
         "This is a template example:\n",
         "\n",
-        "```\n",
+        "```yaml\n",
         "SpaceInvadersNoFrameskip-v4:\n",
         "  env_wrapper:\n",
         "    - stable_baselines3.common.atari_wrappers.AtariWrapper\n",

notebooks/unit5/unit5.ipynb

@@ -324,7 +324,7 @@
         "\n",
         "We'll give you here a first version of this config (to copy and paste into your `SnowballTarget.yaml file`), **but you should modify it**.\n",
         "\n",
-        "```\n",
+        "```yaml\n",
         "behaviors:\n",
         "  SnowballTarget:\n",
         "    trainer_type: ppo\n",