Updated small typo in pretty_print() function

slides/cours_15.md

@@ -707,7 +707,7 @@ void pretty_print(tree_t tree, int n) {
             printf(" ");
         }
         printf("%d\n", tree->info);
-        pretty_print(tree->left);
+        pretty_print(tree->left, n+1);
     }
 }
 ```