Make visibleTodos a required property in example (ref reduxjs#1086)

Author: omnidan

examples/todos-with-undo/containers/App.js

@@ -33,7 +33,7 @@ App.propTypes = {
   visibleTodos: PropTypes.arrayOf(PropTypes.shape({
     text: PropTypes.string.isRequired,
     completed: PropTypes.bool.isRequired
-  })),
+  }).isRequired).isRequired,
   visibilityFilter: PropTypes.oneOf([
     'SHOW_ALL',
     'SHOW_COMPLETED',