[ci skip] Fix form_with model binding example in guides

This commit fixes an inconsistency in the Form Helpers guide where the example was using Book.find(42) while demonstrating a form for creating a new book.

Issue details:
- The example shows a form intended for creating a new book (action='/books' with POST method)
- However, it was incorrectly binding to an existing book with Book.find(42)
- The submit button text is 'Create Book', confirming it's a creation form
- The guide explicitly states 'the following form to create a new book'

Changes made:
- Changed @book = Book.find(42) to @book = Book.new
- Updated the example output to show a new book object with nil values

This change improves documentation accuracy by ensuring the model object matches the form's purpose and behavior. This helps prevent confusion for developers learning Rails form helpers.

Author: porporpor020202

guides/source/form_helpers.md

@@ -249,8 +249,8 @@ The `form_with` helper has a `:model` option that allows you to bind the form bu
 For example, if we have a `@book` model object:
 
 ```ruby
-@book = Book.find(42)
-# => #<Book id: 42, title: "Walden", author: "Henry David Thoreau">
+@book = Book.new
+# => #<Book id: nil, title: nil, author: nil>
 ```
 
 And the following form to create a new book: