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ARROW-16500: [Release][R] Don't use GNU sed extension for r/NEWS.md update

We should not '0,/.../' syntax for updating r/NEWS.md because it's a
GNU sed extension. If we use it, our release script doesn't work on
macOS that uses BSD sed.

Closes #13095 from kou/release-r-news-update

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>