clarify deletion behavior of output-dir (closes #13896)

[petrelharp]

This is a minor (typo-level) PR clarifying this potentially destructive side effect of quarto render, as discussed in #13896.

Checklist

I have (if applicable):

• NA filed a contributor agreement. this is typo-level, so IIUC I can skip this.
• referenced the GitHub issue this PR closes
• NA updated the appropriate changelog in the PR
• ensured the present test suite passes
• NA added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR

[posit-snyk-bot]

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