Divider with subheader example update (flutter#65944)

* Divider with subheader example update

* fixed issues

* Addressed comments

* fixed issues

Author: JoseAlba

packages/flutter/lib/src/material/divider.dart

@@ -49,15 +49,27 @@ import 'theme.dart';
 ///           ),
 ///         ),
 ///         const Divider(
-///           color: Colors.black,
 ///           height: 20,
 ///           thickness: 5,
 ///           indent: 20,
-///           endIndent: 0,
+///           endIndent: 20,
+///         ),
+///         // Subheader example from Material spec.
+///         // https://material.io/components/dividers#types
+///         Container(
+///           padding: const EdgeInsets.only(left: 20),
+///           child: Align(
+///             alignment: AlignmentDirectional.centerStart,
+///             child: Text(
+///               'Subheader',
+///               style: Theme.of(context).textTheme.caption,
+///               textAlign: TextAlign.start,
+///             ),
+///           ),
 ///         ),
 ///         Expanded(
 ///           child: Container(
-///             color: Colors.blue,
+///             color: Theme.of(context).colorScheme.primary,
 ///             child: const Center(
 ///               child: Text('Below'),
 ///             ),