[R] Update flight.Rmd vignette

[asfimport]

Proposed changes:

• "we" -> "you"
• tiny bit more content in the intro about what flight is and why you might use it
• link to the dplyr vignette

Reporter: Nicola Crane / @thisisnic
Assignee: Nicola Crane / @thisisnic

PRs and other links:

• GitHub Pull Request #10943

_{Note: This issue was originally created as ARROW-13402. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 10943
#10943