Update backtracking/subset_sum.cpp

Co-authored-by: David Leal <halfpacho@gmail.com>

backtracking/subset_sum.cpp

@@ -102,6 +102,6 @@ static void test() {
  * @returns 0 on exit
  */
 int main() {
-    test();  // execute the test
+    test();  // run self-test implementations
     return 0;
 }