Fix OnChild dartdoc typo

Author: daohoangson

packages/core/lib/src/core_data.dart

@@ -41,7 +41,7 @@ typedef DefaultStyles = StylesMap Function(BuildTree tree);
 /// ```dart
 /// BuildOp.v1(
 ///   onChild: (tree, subTree) {
-///     if (!subTree.element.parent != tree.element) return;
+///     if (subTree.element.parent != tree.element) return;
 ///     subTree.doSomething();
 ///   },
 /// );