[Typo]: index argument of the map function is not used, so better to omit it as the second optional argument.

### Summary

In the 'Avoid duplication in state' part of this page, 'index' is used as the second argument in the map function, but never used.

### Page

https://react.dev/learn/choosing-the-state-structure

### Details

Remove the 'index' argument from the map function, as it is not used.