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ARROW-13400 [R] Update fs.Rmd (Working with S3) vignette

Just a few updates and fixes to rough edges according to the notes in ARROW-13400! In particular,

- Added a section on using `proxy_options`
- Added that you can use `$ls()` to view a directory listing (I found this useful when testing the S3 proxy server stuff)

Closes #11729 from paleolimbot/r-s3-vignette

Authored-by: Dewey Dunnington <dewey@fishandwhistle.net>
Signed-off-by: Jonathan Keane <jkeane@gmail.com>