Change URLs that refer to master branch to main.

GitHub redirects `master` to `main` so these URLs aren't broken, but we it's still good practice to update them to the current, canonical form.

PiperOrigin-RevId: 465297038
Change-Id: Icdd515727305da40fa7e4954bcbf90b06409ca38

Author: saran-t

README.md

@@ -5,7 +5,7 @@ Learning environments, using MuJoCo physics.
 
 An **introductory tutorial** for this package is available as a Colaboratory
 notebook:
-[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/deepmind/dm_control/blob/master/tutorial.ipynb)
+[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/deepmind/dm_control/blob/main/tutorial.ipynb)
 
 ## Overview
 

setup.py

@@ -182,7 +182,7 @@ def is_excluded(s):
 Learning environments, using MuJoCo physics.
 
 An **introductory tutorial** for this package is available as a Colaboratory
-notebook: [Open In Google Colab](https://colab.research.google.com/github/deepmind/dm_control/blob/master/tutorial.ipynb).
+notebook: [Open In Google Colab](https://colab.research.google.com/github/deepmind/dm_control/blob/main/tutorial.ipynb).
 """,
     long_description_content_type='text/markdown',
     author='DeepMind',

tutorial.ipynb

@@ -8,7 +8,7 @@
       "source": [
         "# **`dm_control` tutorial**\n",
         "\n",
-        "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/deepmind/dm_control/blob/master/tutorial.ipynb)\n",
+        "[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/github/deepmind/dm_control/blob/main/tutorial.ipynb)\n",
         "\n",
         "\n",
         "\n",