Improve child_builder add_child documentation slightly (#16663)

A small documentation improvement. The description was copied from
insert_children. I changed the documentation to be singular instead of
plural when referring to the child in add_child.

# Objective

- The description was copied from insert_children and still refers to
the child being added as plural children

## Solution

- Description now has child in singular form.

## Testing

- N/A

---------

Co-authored-by: IQuick 143 <IQuick143cz@gmail.com>

crates/bevy_hierarchy/src/child_builder.rs

@@ -385,8 +385,8 @@ pub trait BuildChildren {
 
     /// Adds a single child.
     ///
-    /// If the children were previously children of another parent, that parent's [`Children`] component
-    /// will have those children removed from its list. Removing all children from a parent causes its
+    /// If the child was previously the child of another parent, that parent's [`Children`] component
+    /// will have the child removed from its list. Removing all children from a parent causes its
     /// [`Children`] component to be removed from the entity.
     ///
     /// # Panics