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 a273bc9)

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

Author: cla7aye15I4nd

Misc/NEWS.d/next/Library/2025-12-22-22-36-21.gh-issue-122431.9E3085.rst

@@ -0,0 +1 @@
+Corrected the error message in :func:`readline.append_history_file` to state that ``nelements`` must be non-negative instead of positive.

Modules/readline.c

@@ -353,7 +353,7 @@ readline_append_history_file_impl(PyObject *module, int nelements,
 {
     if (nelements < 0)
     {
-        PyErr_SetString(PyExc_ValueError, "nelements must be positive");
+        PyErr_SetString(PyExc_ValueError, "nelements must be non-negative");
         return NULL;
     }