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

Open the Guide Schematics page at https://material.angular.io/guide/schematics and reproduce the issue by reading through to the final “Material 3 Theme schematic” section. Trace the Guide Content highlighting behavior and verify that the final entry becomes highlighted when its content is visible, without changing the highlighting of earlier entries.

Written by the indexing model from the issue text.