corrected typo

slides/boucles_conditions.md

@@ -22,7 +22,7 @@ int main() {
     res += i;
   }
   printf("For = %d, the result is %d\n", n, res);
-  if (sum != n * (n+1) / 2) {
+  if (res != n * (n+1) / 2) {
     printf("Error: the answer is wrong.\n");
     return EXIT_FAILURE;
   }