[3.11] gh-86404: [doc] Fix missing space in idle documentation. (GH-94366) (GH-94369)

This should have been spotted by sphinx-lint, tracked here:

https://github.com/sphinx-contrib/sphinx-lint/issues/39
(cherry picked from commit bc7f6fcdf14da32a8f7816ace800a6a91dc1554f)


Co-authored-by: Julien Palard <julien@palard.fr>

Automerge-Triggered-By: GH:JulienPalard