Use a more restrictive sed pattern

Author: henrymercer

.github/update-release-branch.py

@@ -277,7 +277,7 @@ def main():
 
     # Migrate the changelog notes from v2 version numbers to v1 version numbers
     print('Migrating changelog notes from v2 to v1')
-    subprocess.run(['sed', '-i', 's/## 2\./## 1\./g', 'CHANGELOG.md'])
+    subprocess.run(['sed', '-i', 's/^## 2\./## 1./g', 'CHANGELOG.md'])
 
     # Amend the commit generated by `npm version` to update the CHANGELOG
     run_git('add', 'CHANGELOG.md')