Add a MDN link for landmark (#1455)

Author: Jinjiang

src/guide/best-practices/accessibility.md

@@ -92,7 +92,7 @@ Users can navigate an application through headings. Having descriptive headings
 
 ### Landmarks
 
-Landmarks provide programmatic access to sections within an application. Users who rely on assistive technology can navigate to each section of the application and skip over content. You can use [ARIA roles](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles) to help you achieve this.
+[Landmarks](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/landmark_role) provide programmatic access to sections within an application. Users who rely on assistive technology can navigate to each section of the application and skip over content. You can use [ARIA roles](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles) to help you achieve this.
 
 | HTML            | ARIA Role            | Landmark Purpose                                                                                                 |
 | --------------- | -------------------- | ---------------------------------------------------------------------------------------------------------------- |