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

[asfimport]

Proposed changes:

• "we" -> "you"
• "See the help for FileSystem" -> "See help(FileSystem)"
• Bullet points in "authentication" section for the parameters
• Any other obvious improvements or updates based on new functionality

Reporter: Nicola Crane / @thisisnic
Assignee: Dewey Dunnington / @paleolimbot

PRs and other links:

• GitHub Pull Request #11729

_{Note: This issue was originally created as ARROW-13400. Please see the migration documentation for further details.}

[asfimport]

Jonathan Keane / @jonkeane:
Issue resolved by pull request 11729
#11729