[3.14] gh-122431: Correct the non-negative error message in `readline.append_history_file` (GH-143075) (GH-143090)

gh-122431: Correct the non-negative error message in `readline.append_history_file` (GH-143075)

"positive" -> "non-negative", since zero is included.
(cherry picked from commit a273bc99d2ff853f59ee6da4d897b1be72a03975)

Co-authored-by: Zheng Yu <dataisland@outlook.com>