Make visibleTodos a required property

Since TodoList's todos property is required, visibleTodos should be
required too.

Author: mjethani

docs/basics/ExampleTodoList.md

@@ -155,7 +155,7 @@ App.propTypes = {
   visibleTodos: PropTypes.arrayOf(PropTypes.shape({
     text: PropTypes.string.isRequired,
     completed: PropTypes.bool.isRequired
-  })),
+  }).isRequired).isRequired,
   visibilityFilter: PropTypes.oneOf([
     'SHOW_ALL',
     'SHOW_COMPLETED',

docs/basics/UsageWithReact.md

@@ -343,7 +343,7 @@ App.propTypes = {
   visibleTodos: PropTypes.arrayOf(PropTypes.shape({
     text: PropTypes.string.isRequired,
     completed: PropTypes.bool.isRequired
-  })),
+  }).isRequired).isRequired,
   visibilityFilter: PropTypes.oneOf([
     'SHOW_ALL',
     'SHOW_COMPLETED',