corrected typo

slides/cours_11.md

@@ -336,7 +336,7 @@ bool stack_is_empty(stack s); // return NULL == s.top;
 void stack_push(stack *s, int val) {
     element *elem = malloc(sizeof(*elem));
     elem->data = val;
-    elem->next = *s;
+    elem->next = s->top;
     s->top = elem;
 }
 ```