ARROW-15082: [R] Clean up one more duration mapping entry
ARROW-15082: [R] Clean up one more duration mapping entry

The PR updates the 'Get Started' vignette that should have been updated to reflect the mapping of the duration type to R's difftime in #11850 / ARROW-14941 but wasn't.

Closes #11948 from paleolimbot/r-duration-in-vignette

Authored-by: Dewey Dunnington <918f18460f0c7572f85895a6a65dbb2d51424a08@fishandwhistle.net>
Signed-off-by: Jonathan Keane <28d17c45cab2bb8b25bea44a915713e26ce37e1b@gmail.com>

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