message: Update RELEASE_GUIDE
diff: @@ -50,13 +50,13 @@ Do not push to origin yet.
  git cherry-pick rc/$RELEASE
  ```
  -Update the headers. The "Next release" header becomes a header for $NEXT_RELEASE, and a new "Next
  -release" header is added.
  +Update the headers. The "main branch" header becomes a header for $NEXT_RELEASE, and a new "main
  +branch" header is added.
  For example, this:
  ```
  -## Next release (main branch)
  +## main branch
  - foo
  - bar
  