Update forms_advanced.md (#28627)

Fix typo

On Rails Guide, building-complex-forms is section 9

Co-authored-by: Kevin <[email protected]>

ruby_on_rails/advanced_forms_and_activerecord/forms_advanced.md

@@ -176,7 +176,7 @@ Sometimes Rails helper methods will do it for you, but make sure you know what y
 <div class="lesson-content__panel" markdown="1">
 
   1. Read the Rails Guide on Forms section 5, which covers [populating a form with a collection of objects](https://guides.rubyonrails.org/form_helpers.html#choices-from-a-collection-of-arbitrary-objects).
-  1. Read the Same Rails Guide on Forms section 10, which covers [accepting nested form data](http://guides.rubyonrails.org/form_helpers.html#building-complex-forms).
+  1. Read the Same Rails Guide on Forms section 9, which covers [accepting nested form data](http://guides.rubyonrails.org/form_helpers.html#building-complex-forms).
   1. Read the Same Rails Guide on Forms section 8, which covers the [parameter conventions for nested forms](https://guides.rubyonrails.org/form_helpers.html#form-input-naming-conventions-and-params-hash).
   1. Read this blog post from [Peter Rhoades on working with nested forms](https://www.createdbypete.com/2014/04/04/working-with-nested-forms-and-a-many-to-many-association-in-rails-4.html). The example covers a lot of the things we've gone over so far, so follow along. Also note how he does the allowing of nested attributes in Rails 4.