chore: fix workflow docs.yml

[kenjis]

See https://github.com/codeigniter4/shield/actions/runs/4454434810/jobs/7823617312

Run mkdocs gh-deploy --force
INFO     -  Cleaning site directory
INFO     -  Building documentation to directory: /home/runner/work/shield/shield/site
INFO     -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
  - banning_users.md
  - forcing_password_reset.md
INFO     -  Documentation built in 0.46 seconds
WARNING  -  Version check skipped: No version specified in previous deployment.
INFO     -  Copying '/home/runner/work/shield/shield/site' to 'gh-pages' branch and pushing to GitHub.
remote: Permission to codeigniter4/shield.git denied to github-actions[bot].
fatal: unable to access 'https://github.com/codeigniter4/shield/': The requested URL returned error: 403

Ref https://squidfunk.github.io/mkdocs-material/publishing-your-site/#github-pages