bpo-46059: Clarify pattern-matching example in "control flow" docs (GH-30079)

The "Color" example in the pattern-matching section of the "control flow" documentation is not immediately runnable, leading to confusion.

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
(cherry picked from commit 1cbb88736c32ac30fd530371adf53fe7554be0a5)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>