README.md: replace master w/main

for aomedia.googlesource.com refs; main is the preferred branch now and
pushing to master will be blocked from submitting

Bug: b/204460717
Bug: aomedia:3223
Change-Id: I5b306bc2b4bf15c30689ee8e8e0cb5c42b7b76ef
(cherry picked from commit 77165f5de35d60e2cf6ac239a88ea70a889cc8c8)

README.md

diff --git a/README.md b/README.md
index 118fccc..2e5539b 100644
--- a/README.md
+++ b/README.md
@@ -527,7 +527,7 @@
 
 The coding style used by this project is enforced with clang-format using the
 configuration contained in the
-[.clang-format](https://chromium.googlesource.com/webm/aom/+/master/.clang-format)
+[.clang-format](https://chromium.googlesource.com/webm/aom/+/main/.clang-format)
 file in the root of the repository.
 
 You can download clang-format using your system's package manager, or directly
@@ -614,7 +614,7 @@
 The command line to upload your patch looks like this:
 
 ~~~
-    $ git push https://aomedia-review.googlesource.com/aom HEAD:refs/for/master
+    $ git push https://aomedia-review.googlesource.com/aom HEAD:refs/for/main
 ~~~
 
 ### Incorporating reviewer comments {#incorporating-reviewer-comments}