Replace deprecated Parent with ChildOf in relationships migration guide (#2070)

Author: StudioLE

release-content/0.16/migration-guides/17398_Relationships_nonfragmenting_onetomany.md

@@ -1,5 +1,5 @@
 - Replace `ChildBuilder` with `ChildSpawnerCommands`.
-- Replace calls to `.set_parent(parent_id)` with `.insert(Parent(parent_id))`.
+- Replace calls to `.set_parent(parent_id)` with `.insert(ChildOf(parent_id))`.
 - Replace calls to `.replace_children()` with `.remove::<Children>()` followed by `.add_children()`. Note that you’ll need to manually despawn any children that are not carried over.
 - Replace calls to `.despawn_recursive()` with `.despawn()`.
 - Replace calls to `.despawn_descendants()` with `.despawn_related::<Children>()`.