Fix appearance and add "Open in Colab"

PiperOrigin-RevId: 448540641
Change-Id: I9e5adfe06ff299f737b995e985a11594b12012e2

Author: DeepMind

dm_control/mujoco/tutorial.ipynb

@@ -6,8 +6,11 @@
         "id": "MpkYHwCqk7W-"
       },
       "source": [
-        "# \u003cdiv align=\"center\"\u003e**MuJoCo** tutorial\u003c/div\u003e\n",
-        "### \u003cdiv align=\"center\"\u003eusing `dm_control` Python bindings\u003c/div\u003e\n",
+        "# **MuJoCo** tutorial\n",
+        "### using `dm_control` Python bindings\n",
+        "\n",
+        "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/deepmind/dm_control/blob/main/dm_control/mujoco/tutorial.ipynb)\n",
+        "\n",
         "\n",
         "\n",
         "\n",