docs: fix singular table names in API guide's withAuthorInfo() method

[AlexRasch]

Description
The BookModel::withAuthorInfo() method in the Controller guide uses singular table names (book, author) instead of the plural names (books, authors) created in the Database Setup migration. This updates the method to use the correct plural table names for consistency across the API guide.

Fixes #10057

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[mergeable]

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[michalsn]

@AlexRasch Please run composer cs-fix and send the updated code.

[neznaika0]

We talked about it. Many people refer to tables in the singular. For the API too. In the framework, it is usually in the plural, so there were disagreements.

[michalsn]

The fix is valid. Since the tables were created using plural names, we should refer to them accordingly. This is part of the same guide. See the linked issue.