fix typo in relation manual
@@ -922,7 +922,7 @@ ecs_entity_t Bob = ecs_new_id(world);
ecs_entity_t Alice = ecs_new_id(world);
ecs_add_pair(world, Bob, MarriedTo, Alice); // Also adds (MarriedTo, Bob) to Alice
```
-```c
+```cpp
auto MarriedTo = world.entity().add(flecs::Symmetric);
auto Bob = ecs.entity();
auto Alice = ecs.entity();
