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MINOR: [R] Fix lint error (#38170)


### Rationale for this change

The CI job is failing on Windows after #38002. This was masked in review because the Windows job was failing for another reason (which is also now fixed).

### What changes are included in this PR?

The error is fixed.

### Are these changes tested?

Covered by existing tests

### Are there any user-facing changes?

No

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>