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MINOR: [R] Add missing PR num to news.md item (#43811)

### Rationale for this change

We normally link to somewhere to give the user more context on news items. I noticed the link was missing for this one.

### What changes are included in this PR?

Added PR number to news item.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>