run-on-specific-branches-or-tags1: terminology fix

[wh0]

Why:

In this line, there isn't really such thing as "the undefined Git ref." Rather "defined" and "undefined" describe the branches/tags filters when you define only one of them.

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

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• [ ] A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• [x] The changes in this PR meet the docs fundamentals that are required for all content.
• [ ] All CI checks are passing and the changes look good in the review environment.

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Source	Review	Production	What Changed
actions/how-tos/write-workflows/choose-when-workflows-run/trigger-a-workflow.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	from reusable
actions/reference/workflows-and-actions/workflow-syntax.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	from reusable

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[Sharra-writes]

@wh0 Thanks for opening a PR! Copilot tells me this is correct, and I would be inclined to trust it since this is a small change, if it hadn't just completely made up a citation for me. So. I think I'm going to verify with an actual person.

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