docs: replaces 'Not available' with 'search' (#2604)

I believe it should point to the search element, as it already defines a role landmark: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/search

src/guide/best-practices/accessibility.md

@@ -123,7 +123,7 @@ Users can navigate an application through headings. Having descriptive headings
 | main            | role="main"          | The main or central content of the document.                                                                     |
 | footer          | role="contentinfo"   | Information about the parent document: footnotes/copyrights/links to privacy statement                           |
 | aside           | role="complementary" | Supports the main content, yet is separated and meaningful on its own content                                    |
-| _Not available_ | role="search"        | This section contains the search functionality for the application                                               |
+| search          | role="search"        | This section contains the search functionality for the application                                               |
 | form            | role="form"          | Collection of form-associated elements                                                                           |
 | section         | role="region"        | Content that is relevant and that users will likely want to navigate to. Label must be provided for this element |