replace universal quantifier with existential in knitr compatibility claim

[cscheid]

For consistency with "highly" (vs "perfectly"), "generally" (vs "always"), and "should" (vs "must"), let's use "an" (vs "any").

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[cscheid]

Sure - this isn't high priority and would just get eventually merged to main, but we can do an extra PR there.

[cderv]

would just get eventually merged to main

This PR is against the prerelease branch, so I was thinking we could do it to main instead, and then we would have auto-commit to prerelease with the automated PR that would be created.

So usually, when something can be fixed or have more precision, then I do it in main through PR to get auto port to prerelease branch. That way users benefits right now for the improved doc.

Both ways are good - just wanted to share this practice.