[3.13] marshal docs: Remove reference to "Sun" (GH-119161) (#119167)

Nobody has been using a Sun machine for a long time. When I saw
this sentence in a lightning talk just now, I thought it was talking
about sending Python code on a spacecraft.
(cherry picked from commit 697465ff88e49d98443025474e5b534adfba2cb0)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>