arrow

ARROW-15103: [Documentation][C++] Error building docs: "arrow/cpp/src/arrow/csv/options.h:182: error: Found unknown command '\r' "

This PR fixes a minor error in the C++ documentation which prevents it from building.

(I would have just submitted the PR as [MINOR] but I'd already opened the ticket!)

Closes #11952 from thisisnic/ARROW-15103_cpp_docs

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>