Guide Content list element is not highlighting Last index element even we are reading last content

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

I am going through https://material.angular.io/guide/schematics
and I noticed following things
1) when I reading left content by content it highlights current topic name in right side list
but its not working for last content
- when i am reding last content **Material 3 Theme schematic** but right side highlighter showing only on **Drag and Drop schematic** instead of **Material 3 Theme schematic**

### Reproduction

I am going through https://material.angular.io/guide/schematics
and I noticed following things
1) when I reading left content by content it highlights current topic name in right side list
but its not working for last content
- when i am reding last content **Material 3 Theme schematic** but right side highlighter showing only on **Drag and Drop schematic** instead of **Material 3 Theme schematic**

### Expected Behavior

when I am reading last content in list then only last element is not highlighted in **Guide Content** list

### Actual Behavior

when I am reading last content in list then only last element is should be highlighted in **Guide Content** list

### Environment

https://material.angular.io/guide/schematics の Guide Schematics ページを開き、最後の「Material 3 Theme schematic」セクションまで読み進めて問題を再現します。Guide Content のハイライト動作を追跡し、コンテンツが表示されているときに最後のエントリがハイライトされることを、以前のエントリのハイライトを変更せずに確認します。

索引モデルが issue の本文から書いたものです。