Fix name of '\0'. (GH-14222)


'\0' is the NUL byte not NULL.
(cherry picked from commit 7821b4c6d29933511d50bb42255e39790c6abf00)

Co-authored-by: Benjamin Peterson <benjamin@python.org>
diff --git a/Modules/_csv.c b/Modules/_csv.c
index 7eb9d8b..014cbb4 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -826,7 +826,7 @@
             if (c == '\0') {
                 Py_DECREF(lineobj);
                 PyErr_Format(_csvstate_global->error_obj,
-                             "line contains NULL byte");
+                             "line contains NUL");
                 goto err;
             }
             if (parse_process_char(self, c) < 0) {