Fix grammar in Doc/tutorial/controlflow.rst (GH-21885) (#21922)

Automerge-Triggered-By: @csabella
(cherry picked from commit 0be7c216e16f0d459f1c8f6209734c9b2b82fbd4)

Co-authored-by: Denis Ovsienko <denis@ovsienko.info>
1 file changed