Update reference/cstdlib/mblen.md

Co-authored-by: Raclamusi <[email protected]>

Author: akinomyoga

reference/cstdlib/mblen.md

@@ -38,7 +38,7 @@ namespace std {
 int main() {
   std::setlocale(LC_ALL, "ja_JP.UTF-8");
   const char *str = "こんにちは";
-  int result = std::mblen(str, std::strlen(s));
+  int result = std::mblen(str, std::strlen(str));
   std::cout << result << std::endl;
   return 0;
 }