Update readme to reference main branch

Change-Id: I462525311499756b8743f0700e2041576e7b21fd
Reviewed-on: https://fuchsia-review.googlesource.com/c/cobalt/+/525606
Fuchsia-Auto-Submit: Steve Fung <stevefung@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Zach Bush <zmbush@google.com>
diff --git a/README.md b/README.md
index 4c3522a..8a949b8 100644
--- a/README.md
+++ b/README.md
@@ -49,7 +49,7 @@
 - An *embedding* of Cobalt Core into a client-side platform.
   - This allows the code-to-be-measured to easily invoke Cobalt's *Logger* interface.
   - There is an embedding into the *Fuchsia* operating system at
-    [//src/cobalt](https://fuchsia.googlesource.com/fuchsia/+/refs/heads/master/src/cobalt/).
+    [//src/cobalt](https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/src/cobalt/).
   - If you have a Fuchsia checkout, you will find a copy of Cobalt Core has
     been pulled in at `//third_party/cobalt`.
   - As of this writing, Fuchsia is the only embedding of Cobalt Core.
@@ -77,7 +77,7 @@
  the following:
   * Fetches third party code into the `third_party` dir via Git submodules.
   * Fetches the sysroot and puts it in the `sysroot` dir. This uses
-  [CIPD](https://chromium.googlesource.com/chromium/src.git/+/master/docs/cipd.md).
+  [CIPD](https://chromium.googlesource.com/chromium/src.git/+/main/docs/cipd.md).
 
 ## cobaltb.py
 The Python script `cobaltb.py` in the root directory is used to orchestrate
@@ -129,7 +129,7 @@
 - Commit the updated files and write a change message.
 ``` git commit```
 - Upload the commit to Gerrit:
-```git push origin HEAD:refs/for/master```
+```git push origin HEAD:refs/for/main```
 
 See the [Gerrit documentation](https://gerrit-documentation.storage.googleapis.com/Documentation/2.12.3/intro-user.html#upload-change) for more information.
 
@@ -142,6 +142,6 @@
 - Include the patch in the same commit using the `--amend` option:
 ```git commit --amend```
 - Upload the patch to Gerrit:
-```git push origin HEAD:refs/for/master```
+```git push origin HEAD:refs/for/main```