Update Chapter 2.4 "Undoing Things" to mention git rm --cached for unstaging files

[furqansafdar]

There's no existing/similar bug report.

• I have searched the existing issues

This report is about a single actionable bug.

• I'm reporting a single actionable bug

This report is about the ProGit book, version 2, English language.

• This bug is not about a translation or old version

Bug covers book website/pdf

• I confirm the bug is about the book as found on the website/pdf

Problem is present in the Pro Git book on the website?

• This bug also affects the Pro Git book as published on the website.

Which version of the book is affected?

I don't know

Describe the bug:

While reading through the book, I noticed that Chapter 2, Section 4 ("Undoing Things") heavily relies on git reset HEAD ... to demonstrate how to unstage a file. While git reset still works for backwards compatibility, modern versions of Git (v2.23+) now explicitly output (use "git rm --cached ..." to unstage) when running git status. To ensure the text aligns with modern terminal hints and best practices, it would be highly beneficial to update this section to prioritize git restore --staged as the primary method for unstaging, while keeping a brief note about legacy git reset functionality.

Expected behavior:

Commands to be updated in the textbook