minor #1874 Fix the example with Expression Language (seb-jean)

This PR was merged into the 2.x branch.

Discussion
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Fix the example with Expression Language

`@stof` said in symfony/symfony-docs#17905:
> When evaluating the expression, the ExpressionLanguage does not have access to private properties of the object, as it runs from its outside. And contrary to Twig, ExpressionLanguage does not have the magic . operator that tries to find a getter when it cannot use the property.

I then improved the documentation :).

Commits
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15c6786 Fix the example with Expression Language

Author: kbond

src/Turbo/doc/index.rst

@@ -729,8 +729,8 @@ Example::
 
     use Symfony\UX\Turbo\Attribute\Broadcast;
 
-    #[Broadcast(topics: ['@="book_detail" ~ entity.id', 'books'], template: 'book_detail.stream.html.twig', private: true)]
-    #[Broadcast(topics: ['@="book_list" ~ entity.id', 'books'], template: 'book_list.stream.html.twig', private: true)]
+    #[Broadcast(topics: ['@="book_detail" ~ entity.getId()', 'books'], template: 'book_detail.stream.html.twig', private: true)]
+    #[Broadcast(topics: ['@="book_list" ~ entity.getId()', 'books'], template: 'book_list.stream.html.twig', private: true)]
     class Book
     {
         // ...