MINOR: [R] Fix misalignment in arrow.Rmd vignette
MINOR: [R] Fix misalignment in arrow.Rmd vignette

Closes #12287 from jonkeane/vignette-map-misalignment

Authored-by: Jonathan Keane <28d17c45cab2bb8b25bea44a915713e26ce37e1b@gmail.com>
Signed-off-by: Jonathan Keane <28d17c45cab2bb8b25bea44a915713e26ce37e1b@gmail.com>

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