Update README instructions for running tests

[nathany]

It looks like changing a directory isn't necessary, given the library path is specified in the next line.

❯ cd packages/trpl 
                                                                                          
❯ mdbook test --library-path packages/trpl/target/debug/deps
2025-06-30 10:09:36 [ERROR] (mdbook::utils): Error: Couldn't open SUMMARY.md in "/Users/nathany/Development/rust/books/the-rust-programming-language-manuscript/packages/trpl/src" directory
2025-06-30 10:09:36 [ERROR] (mdbook::utils): 	Caused By: No such file or directory (os error 2)

Works fine without the cd line.

[nathany]

It looks like it may not be this simple. I'll open an issue to discuss instead.

[nathany]

Existing issue #4414