actions-scheduled-workflow-example: fix cron spec

[philpennock]

Why:

The prior cron spec would trigger 4 times a day, at 05:00, 05:30, 17:00, and 17:30. The */30 is "any minute divisible by thirty" which is twice per hour.

So fix the spec to use 30 in place of */30 to match the description.

What's being changed:

reusables.repositories.actions-scheduled-workflow-example : crontab example for GitHub Actions.

Check off the following:

• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
  • https://docs-5572--patch-1.herokuapp.com/en/actions/reference/workflow-syntax-for-github-actions under on.schedule
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[janiceilene]

@philpennock Thanks so much for opening a PR! I'll get this triaged for review ⚡

[shati-patel]

Thank you for fixing this @philpennock! 😃

[github-actions]

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