Update cours_11.md

slides/cours_11.md

@@ -248,7 +248,8 @@ void stack_pop(stack *s, int *val) {
 ```C
 void stack_destroy(stack *s) {
     while (!stack_is_empty(*s)) {
-        int val = stack_pop(s);
+        int val; 
+        stack_pop(s, &val);
     }
 }
 ```