Fix auto-merge workflow

[masa10-f]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run pytest)
• If applicable, add a line to the [unreleased] part of CHANGELOG.md, following keep-a-changelog.
• Format added code by ruff
• Type checking by mypy and pyright
• Make sure the checks (github actions) pass.
• Check that the docs compile without errors (run make html in ./docs/ - you may need to install dependency for sphinx docs, see docs/requirements.txt.)

Then, please fill in below:

Description of the change:

• Fixed the branch name in dependabot-auto-merge.yml, which enables auto-mege of PR opened by the dependabot
• Explicitly specified the PR_URL

[shinich1]

Are you sure about auto merge? There was a case where I had to modify dependabot's PR before safely merging due to breaking changes in dependencies (pyzx, numpy).

[masa10-f]

@shinich1
--auto mode will merge if and only if all the CIs are passed according to the document. Therefore, we will probably be able to modify depandabot's PR if necessary