bpo-45799: [Doc] improve confusing sentence in __main__.rst (GH-29546)

I was reading this bit last night and thought it was a typo. In the light of day, I realized it wasn't *technically* a typo, but definitely confusing wording. This PR fixes the confusing sentence.

Automerge-Triggered-By: GH:ericvsmith
(cherry picked from commit f8da00ef04fdadf7cd9821e8ec4b317ecf3ed663)

Co-authored-by: Jack DeVries <jdevries3133@gmail.com>