Updated test case documentation

docs/test_cases/t20018.md

@@ -27,16 +27,23 @@ namespace t20018 {
 template <int N> struct Factorial {
     static const int value = N * Factorial<N - 1>::value;
 
-    static void print() { Factorial<N - 1>::print(); }
+    static void print(int answer) { Factorial<N - 1>::print(answer); }
 };
 
 template <> struct Factorial<0> {
     static const int value = 1;
 
-    static void print() { std::cout << "Hello world\n"; }
+    static void print(int answer)
+    {
+        std::cout << "The answer is " << answer << "\n";
+    }
 };
 
-void tmain() { Factorial<5>::print(); }
+template <typename T, int N = T::value> struct Answer {
+    static void print() { T::print(N); }
+};
+
+void tmain() { Answer<Factorial<5>>::print(); }
 }
 }
 ```