Tweak syntax for the merged branch deletion workflow

[peterzhuamazon]

Description

Tweak syntax for the merged branch deletion workflow

Issues Resolved

#6400

Checklist

• By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin.
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[opensearch-trigger-bot]

The backport to 2.12 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.12 2.12
# Navigate to the new working tree
pushd ../.worktrees/backport-2.12
# Create a new branch
git switch --create backport/backport-6814-to-2.12
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 0c90ded7e4506fd51eb48d004d15bebb5f5fd426
# Push it to GitHub
git push --set-upstream origin backport/backport-6814-to-2.12
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.12

Then, create a pull request where the base branch is 2.12 and the compare/head branch is backport/backport-6814-to-2.12.