chore: migrate default branch from master to main (#849)

diff --git a/docs/index.rst b/docs/index.rst
index 9544259..8a5f13a 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -62,7 +62,7 @@
 details, see `LICENSE`_
 
 .. _LICENSE:
-    https://github.com/GoogleCloudPlatform/google-auth-library-python/blob/master/LICENSE
+    https://github.com/GoogleCloudPlatform/google-auth-library-python/blob/main/LICENSE
 
 Contributing
 ------------
@@ -71,4 +71,4 @@
 for details.
 
 .. _contributing:
-    https://github.com/GoogleCloudPlatform/google-auth-library-python/blob/master/CONTRIBUTING.rst
+    https://github.com/GoogleCloudPlatform/google-auth-library-python/blob/main/CONTRIBUTING.rst