ctypes docs: Fix array-length reference to "non-negative" from "positive" (GH-32097) (GH-32142)

(cherry picked from commit 76f14b0463dc2c53911eaf95e85374e511ba9bcc)

Co-authored-by: Yonatan Goldschmidt <yon.goldschmidt@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>