Update managing-environments-for-deployment.md to clear wording ambiguity

[KennanHunter]

Why:

The previous wording made it sound like the only way to make an environment secret in a public repo was to be on the GitHub free plan. While minor, my sleep deprived eyes took an extra few seconds to parse it correctly.

Closes:

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

Just a wording change.

Check off the following:

• 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 a SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing.

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Content directory changes

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Source	Preview	Production	What Changed
actions/managing-workflow-runs-and-deployments/managing-deployments/managing-environments-for-deployment.md	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10	fpt ghec ghes@ 3.15 3.14 3.13 3.12 3.11 3.10

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fpt: Free, Pro, Team
ghec: GitHub Enterprise Cloud
ghes: GitHub Enterprise Server

[nguyenalex836]

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

[subatoi]

Many thanks @KennanHunter — I agree this is clearer so I'll get this merged for you now 👍

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