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MINOR: [R] fix R dev docs (#37595)



### Rationale for this change

This fixes this failing build: https://github.com/apache/arrow-site/actions/runs/5745111541/job/15572612431#step:9:27

### What changes are included in this PR?

That job is now called `test-ubuntu-22.04-docs` and started running the day after `test-ubuntu-default-docs` stopped running. I wasn't sure if it was possible (or worth it) to extend the task template into the ruby file so I just opted for a code comment for future changes. 

### Are these changes tested?

### Are there any user-facing changes?

dev docs will build again

Lead-authored-by: Sam Albers <sam.albers@gmail.com>
Co-authored-by: Jacob Wujciak-Jens <jacob@wujciak.de>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>