[C++][Docs] Consistent variable names in WriteCSV example

[asfimport]

The CSV output documentation uses options instead of write options, which is inconsistent and unclear.

Reporter: Benson Muite / @bkmgit
Assignee: Benson Muite / @bkmgit

PRs and other links:

• GitHub Pull Request #11228

_{Note: This issue was originally created as ARROW-14116. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 11228
#11228